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import { useCallback, useEffect, useMemo, useRef, useState } from 'react'
import type { GeneratedProblem } from '@/db/schema/session-plans'
// =============================================================================
// Types
// =============================================================================
/**
* Problem context passed as a prop - the single source of truth for what problem
* is currently being worked on. This is computed by the parent component from
* redoState + session plan, eliminating dual sources of truth.
*/
export interface ProblemContext {
/** The problem being solved */
problem: GeneratedProblem
/** Index in the current part (original slot index for redos) */
slotIndex: number
/** Part index in the session */
partIndex: number
}
/**
* Input-level state for a problem attempt.
* This is what the student is actively working on - their answer input.
*/
export interface AttemptInput {
/** The problem being solved */
problem: GeneratedProblem
/** Index in the current part */
slotIndex: number
/** Part index in the session */
partIndex: number
/** When the attempt started */
startTime: number
/** Accumulated time spent paused (ms) - subtract from elapsed time for actual response time */
accumulatedPauseMs: number
/**
* Subset of `accumulatedPauseMs` that came from the tab being hidden /
* backgrounded (visibility layer, #156). Audit-only — recorded on the result
* as `hiddenTimeExcludedMs`; it is already included in `accumulatedPauseMs`.
*/
accumulatedHiddenMs: number
/** User's current answer input */
userAnswer: string
/** Number of times user used backspace or had digits rejected */
correctionCount: number
/** Whether manual submit is required (exceeded auto-submit threshold) */
manualSubmitRequired: boolean
/** Rejected digit to show as red X (null = no rejection) */
rejectedDigit: string | null
}
/**
* Context for help mode - computed when entering help
*/
export interface HelpContext {
/** Index of the term being helped with */
termIndex: number
/** Current running total before this term */
currentValue: number
/** Target value after adding this term */
targetValue: number
/** The term being added */
term: number
}
/**
* Snapshot of an attempt that's animating out during transition
*/
export interface OutgoingAttempt {
key: string
problem: GeneratedProblem
userAnswer: string
result: 'correct' | 'incorrect'
/** Part the outgoing problem belonged to (so the transition can render its
* own vertical/linear format, which may differ from the incoming part). */
partIndex: number
}
/**
* Context for the disambiguation phase - when user's input matches
* an intermediate prefix sum but could also be the start of the final answer
*/
export interface DisambiguationContext {
/** Index of the matched prefix sum */
matchedPrefixIndex: number
/** Term index to show help for if they want it */
helpTermIndex: number
/** When the disambiguation timer started */
timerStartedAt: number
}
/**
* Non-paused phases (used for resumePhase type)
*/
export type ActivePhase =
| { phase: 'loading' }
| { phase: 'inputting'; attempt: AttemptInput }
| {
phase: 'awaitingDisambiguation'
attempt: AttemptInput
disambiguationContext: DisambiguationContext
}
| { phase: 'helpMode'; attempt: AttemptInput; helpContext: HelpContext }
| { phase: 'submitting'; attempt: AttemptInput }
| {
phase: 'showingFeedback'
attempt: AttemptInput
result: 'correct' | 'incorrect'
}
| {
phase: 'transitioning'
outgoing: OutgoingAttempt
incoming: AttemptInput
}
| { phase: 'complete' }
/**
* Discriminated union representing all possible interaction phases.
* Each phase carries exactly the data needed for that phase.
*/
export type InteractionPhase =
| ActivePhase
// Session paused - remembers what phase to return to
| {
phase: 'paused'
resumePhase: ActivePhase
/** When the pause started (used to calculate pause duration on resume) */
pauseStartedAt: number
}
/** Threshold for correction count before requiring manual submit */
export const MANUAL_SUBMIT_THRESHOLD = 2
/** Time to wait before auto-triggering help in ambiguous cases (ms) */
export const AMBIGUOUS_HELP_DELAY_MS = 4000
// =============================================================================
// Helper Functions
// =============================================================================
/**
* Gets the active (non-paused) phase, unwrapping if paused.
*/
export function getActivePhase(phase: InteractionPhase): ActivePhase {
return phase.phase === 'paused' ? phase.resumePhase : phase
}
/**
* Applies a transformation to the active phase, preserving paused wrapper if present.
* If the transform returns null, the phase is unchanged.
*/
export function transformActivePhase(
phase: InteractionPhase,
transform: (active: ActivePhase) => ActivePhase | null
): InteractionPhase {
if (phase.phase === 'paused') {
const newResumePhase = transform(phase.resumePhase)
if (newResumePhase === null) return phase
return {
phase: 'paused',
resumePhase: newResumePhase,
pauseStartedAt: phase.pauseStartedAt,
}
}
const newPhase = transform(phase)
return newPhase === null ? phase : newPhase
}
/**
* Creates a fresh attempt input for a new problem
*/
export function createAttemptInput(
problem: GeneratedProblem,
slotIndex: number,
partIndex: number
): AttemptInput {
return {
problem,
slotIndex,
partIndex,
startTime: Date.now(),
accumulatedPauseMs: 0,
accumulatedHiddenMs: 0,
userAnswer: '',
correctionCount: 0,
manualSubmitRequired: false,
rejectedDigit: null,
}
}
/**
* Returns the attempt an active phase is currently carrying, or null for phases
* that carry none (`loading` / `complete`).
*/
export function getActivePhaseAttempt(active: ActivePhase): AttemptInput | null {
switch (active.phase) {
case 'inputting':
case 'awaitingDisambiguation':
case 'helpMode':
case 'submitting':
case 'showingFeedback':
return active.attempt
case 'transitioning':
return active.incoming
case 'loading':
case 'complete':
return null
}
}
/**
* Adds `ms` to the active phase's current attempt `accumulatedPauseMs`, and —
* when the span was hidden-tab time — its `accumulatedHiddenMs` audit counter.
* No-ops for phases without an attempt or a non-positive span. Preserves the
* paused wrapper is NOT this function's job: callers pass an already-unwrapped
* {@link ActivePhase}.
*
* Extracted from `resume()` so the visibility layer folds hidden time through
* the identical accounting the pause clock uses.
*/
export function addPauseToActivePhase(
active: ActivePhase,
ms: number,
opts: { hidden: boolean } = { hidden: false }
): ActivePhase {
if (ms <= 0) return active
const bump = (attempt: AttemptInput): AttemptInput => ({
...attempt,
accumulatedPauseMs: attempt.accumulatedPauseMs + ms,
accumulatedHiddenMs: opts.hidden ? attempt.accumulatedHiddenMs + ms : attempt.accumulatedHiddenMs,
})
switch (active.phase) {
case 'inputting':
case 'awaitingDisambiguation':
case 'helpMode':
case 'submitting':
case 'showingFeedback':
return { ...active, attempt: bump(active.attempt) }
case 'transitioning':
// Fold into the incoming attempt (mirrors resume()'s existing behavior).
return { ...active, incoming: bump(active.incoming) }
case 'loading':
case 'complete':
return active
}
}
/**
* Folds a hidden span `[hiddenAt, now]` into the active phase's current attempt
* as excluded (hidden) time. The span is clamped to the attempt's own lifetime
* (`max(hiddenAt, attempt.startTime)`) so a hidden interval that straddled a
* problem transition can never drive the new attempt's response time negative.
*/
export function foldHiddenSpanIntoActivePhase(
active: ActivePhase,
hiddenAt: number,
now: number
): ActivePhase {
const attempt = getActivePhaseAttempt(active)
if (!attempt) return active
const effectiveStart = Math.max(hiddenAt, attempt.startTime)
return addPauseToActivePhase(active, now - effectiveStart, { hidden: true })
}
/**
* Computes prefix sums for a problem's terms.
* prefixSums[i] = sum of terms[0..i] (inclusive)
*/
export function computePrefixSums(terms: number[]): number[] {
const sums: number[] = []
let total = 0
for (const term of terms) {
total += term
sums.push(total)
}
return sums
}
/**
* Checks if a digit would be consistent with any prefix sum.
*
* Leading zeros are allowed as a disambiguation mechanism:
* - Typing "0" when answer is empty is allowed (starts a multi-digit entry)
* - Typing "03" signals intent to enter a 2+ digit number ending in 3
* - The numeric value after stripping leading zeros must be consistent
*/
export function isDigitConsistent(
currentAnswer: string,
digit: string,
prefixSums: number[]
): boolean {
const newAnswer = currentAnswer + digit
// Allow leading zeros as disambiguation (e.g., "0" or "03" for 3)
// Strip leading zeros to get the numeric prefix we're building toward
const strippedAnswer = newAnswer.replace(/^0+/, '') || '0'
// If the result is just zeros, allow it (user is typing leading zeros)
if (strippedAnswer === '0' && newAnswer.length > 0) {
// Allow typing zeros as long as we haven't exceeded max answer length
const maxLength = Math.max(...prefixSums.map((s) => s.toString().length))
return newAnswer.length <= maxLength
}
const newAnswerNum = parseInt(strippedAnswer, 10)
if (Number.isNaN(newAnswerNum)) return false
for (const sum of prefixSums) {
const sumStr = sum.toString()
// Check if stripped answer is a prefix of any sum
if (sumStr.startsWith(strippedAnswer)) {
return true
}
}
return false
}
/**
* Result of checking for prefix sum matches
*/
export interface PrefixMatchResult {
/** Index of matched prefix sum (-1 if none) */
matchedIndex: number
/** Whether this is an ambiguous match (could be digit-prefix of final answer) */
isAmbiguous: boolean
/** The term index to show help for (matchedIndex + 1, since we help with the NEXT term) */
helpTermIndex: number
}
/**
* Finds which prefix sum the user's answer matches, if any.
* Also detects ambiguous cases where the input could be either:
* 1. An intermediate prefix sum (user is stuck)
* 2. The first digit(s) of a later prefix sum (user is still typing)
*
* Leading zeros disambiguate and REQUEST help:
* - "3" alone is ambiguous (could be prefix sum 3 OR first digit of 33)
* - "03" is unambiguous - user clearly wants help with prefix sum 3
*/
export function findMatchedPrefixIndex(
userAnswer: string,
prefixSums: number[]
): PrefixMatchResult {
const noMatch: PrefixMatchResult = {
matchedIndex: -1,
isAmbiguous: false,
helpTermIndex: -1,
}
if (!userAnswer) return noMatch
// Leading zeros indicate user is explicitly requesting help for that prefix sum
// "03" means "I want help with prefix sum 3" - this is NOT ambiguous
const hasLeadingZero = userAnswer.startsWith('0') && userAnswer.length > 1
const answerNum = parseInt(userAnswer, 10)
if (Number.isNaN(answerNum)) return noMatch
const finalAnswer = prefixSums[prefixSums.length - 1]
// Check if this is the final answer
if (answerNum === finalAnswer) {
return {
matchedIndex: prefixSums.length - 1,
isAmbiguous: false,
helpTermIndex: -1,
}
}
// Check if user's input matches an intermediate prefix sum
const matchedIndex = prefixSums.findIndex(
(sum, i) => i < prefixSums.length - 1 && sum === answerNum
)
if (matchedIndex === -1) return noMatch
// If they used leading zeros, they're explicitly requesting help - NOT ambiguous
// "03" clearly means "help me with prefix sum 3"
if (hasLeadingZero) {
return {
matchedIndex,
isAmbiguous: false, // Leading zero removes ambiguity - they want help
helpTermIndex: matchedIndex + 1,
}
}
// Check if user's input could be a digit-prefix of ANY later prefix sum
// For example, with prefix sums [2, 3, 33, 43, 44]:
// - "3" matches prefix sum at index 1, but could also be first digit of 33 or 43
// - So "3" is ambiguous
const couldBeLaterPrefixPrefix = prefixSums.slice(matchedIndex + 1).some((laterSum) => {
const laterSumStr = laterSum.toString()
return laterSumStr.startsWith(userAnswer)
})
return {
matchedIndex,
isAmbiguous: couldBeLaterPrefixPrefix,
helpTermIndex: matchedIndex + 1, // Help with the NEXT term after the matched sum
}
}
/**
* Computes help context for a given term index
*/
export function computeHelpContext(terms: number[], termIndex: number): HelpContext {
const sums = computePrefixSums(terms)
const currentValue = termIndex === 0 ? 0 : sums[termIndex - 1]
const targetValue = sums[termIndex]
const term = terms[termIndex]
return { termIndex, currentValue, targetValue, term }
}
// =============================================================================
// Hook
// =============================================================================
export interface InitialProblemData {
problem: GeneratedProblem
slotIndex: number
partIndex: number
}
/**
* Active problem context with a stable key for change detection.
* The key allows the hook to detect when the problem truly changes,
* not just when references change.
*/
export interface ActiveProblem {
problem: GeneratedProblem
slotIndex: number
partIndex: number
/** Stable identifier for this problem instance - changes when the problem changes */
key: string
}
export interface UseInteractionPhaseOptions {
/** Initial problem to hydrate with (for SSR) */
initialProblem?: InitialProblemData
/**
* Active problem context - derived from redoState + session plan.
* When the key changes, the hook automatically loads the new problem.
* This is the single source of truth for which problem is being worked on.
*/
activeProblem?: ActiveProblem | null
/** Called when auto-submit threshold is exceeded */
onManualSubmitRequired?: () => void
/**
* When true (LINEAR parts), the abacus is put away entirely: never reject a
* digit, never enter awaitingDisambiguation, and never enter help mode. The
* student just types the final answer to the number sentence.
*/
disableAbacusScaffolding?: boolean
}
export interface UseInteractionPhaseReturn {
// Current phase
phase: InteractionPhase
// Extracted state from phase (single source of truth)
/** Current attempt being worked on (null in loading/complete phases) */
attempt: AttemptInput | null
/** Help context when in helpMode (null otherwise) */
helpContext: HelpContext | null
/** Outgoing attempt during transition animation (null otherwise) */
outgoingAttempt: OutgoingAttempt | null
// Derived predicates for UI
/** Can we accept keyboard/keypad input? */
canAcceptInput: boolean
/** Should the problem display show as completed? */
showAsCompleted: boolean
/** Should the help overlay be shown? */
showHelpOverlay: boolean
/** Should the input area (keypad/submit) be shown? */
showInputArea: boolean
/** Should the feedback message be shown? */
showFeedback: boolean
/** Is the input box focused? */
inputIsFocused: boolean
/** Are we currently in the transitioning phase? */
isTransitioning: boolean
/** Are we currently paused? */
isPaused: boolean
/** Are we currently submitting? */
isSubmitting: boolean
// Computed values (only valid when attempt exists)
/** Prefix sums for current problem */
prefixSums: number[]
/** Full prefix match result with ambiguity info */
prefixMatch: PrefixMatchResult
/** Matched prefix index (-1 if none) - shorthand for prefixMatch.matchedIndex */
matchedPrefixIndex: number
/** Can the submit button be pressed? */
canSubmit: boolean
/** Should auto-submit trigger? */
shouldAutoSubmit: boolean
// Ambiguous prefix state
/** Term index to show "need help?" prompt for (-1 if not in awaitingDisambiguation phase) */
ambiguousHelpTermIndex: number
// Actions
/** Load a new problem (loading → inputting) */
loadProblem: (problem: GeneratedProblem, slotIndex: number, partIndex: number) => void
/** Handle digit input */
handleDigit: (digit: string) => void
/** Handle backspace */
handleBackspace: () => void
/** Enter help mode (inputting → helpMode) */
enterHelpMode: (termIndex: number) => void
/** Exit help mode (helpMode → inputting) */
exitHelpMode: () => void
/** Clear the current answer (used after help overlay transition completes) */
clearAnswer: () => void
/** Set the answer to a specific value (used when showing target reached value) */
setAnswer: (value: string) => void
/** Submit answer (inputting/helpMode → submitting) */
startSubmit: () => void
/** Handle submit result (submitting → showingFeedback) */
completeSubmit: (result: 'correct' | 'incorrect') => void
/** Start transition to next problem (showingFeedback → transitioning) */
startTransition: (
nextProblem: GeneratedProblem,
nextSlotIndex: number,
nextPartIndex?: number
) => void
/** Complete transition (transitioning → inputting) */
completeTransition: () => void
/** Clear to loading state */
clearToLoading: () => void
/** Mark session as complete */
markComplete: () => void
/** Pause session (* → paused) */
pause: () => void
/** Resume session (paused → resumePhase) */
resume: () => void
/** Is the session complete? */
isComplete: boolean
}
export function useInteractionPhase(
options: UseInteractionPhaseOptions = {}
): UseInteractionPhaseReturn {
const { initialProblem, activeProblem, onManualSubmitRequired, disableAbacusScaffolding } =
options
// Initialize state with problem if provided (for SSR hydration)
const [phase, setPhase] = useState<InteractionPhase>(() => {
if (initialProblem) {
const attempt = createAttemptInput(
initialProblem.problem,
initialProblem.slotIndex,
initialProblem.partIndex
)
return { phase: 'inputting', attempt }
}
return { phase: 'loading' }
})
// Live mirror of `phase` for event handlers that must read the current phase
// without re-subscribing (the visibility layer). Set during render — safe.
const phaseRef = useRef(phase)
phaseRef.current = phase
// Live mirror of the abacus-scaffolding-disabled flag (LINEAR parts) so the
// memoized handleDigit/enterHelpMode callbacks read the current value without
// being re-created when it toggles. Set during render — safe.
const disableScaffoldingRef = useRef(disableAbacusScaffolding)
disableScaffoldingRef.current = disableAbacusScaffolding
// Visibility layer (#156): when the tab is hidden/backgrounded we record the
// instant here and fold the elapsed span into the current attempt on return,
// the identical subtraction resume() performs. `null` = not currently hidden.
// The pause clock and this hidden clock form a disjoint union (see pause/
// resume + the visibility effect below) so time is never double-counted.
const hiddenAtRef = useRef<number | null>(null)
// ==========================================================================
// Derived State
// ==========================================================================
// Extract attempt from phase if available
const attempt = useMemo((): AttemptInput | null => {
switch (phase.phase) {
case 'inputting':
case 'awaitingDisambiguation':
case 'helpMode':
case 'submitting':
case 'showingFeedback':
return phase.attempt
case 'transitioning':
return phase.incoming
case 'paused': {
// Recurse into resumePhase
const inner = phase.resumePhase
if (
inner.phase === 'inputting' ||
inner.phase === 'awaitingDisambiguation' ||
inner.phase === 'helpMode' ||
inner.phase === 'submitting' ||
inner.phase === 'showingFeedback'
) {
return inner.attempt
}
if (inner.phase === 'transitioning') {
return inner.incoming
}
return null
}
default:
return null
}
}, [phase])
const prefixSums = useMemo(() => {
if (!attempt) return []
return computePrefixSums(attempt.problem.terms)
}, [attempt])
const prefixMatch = useMemo((): PrefixMatchResult => {
if (!attempt) return { matchedIndex: -1, isAmbiguous: false, helpTermIndex: -1 }
return findMatchedPrefixIndex(attempt.userAnswer, prefixSums)
}, [attempt, prefixSums])
// Convenience shorthand - most callers only need the index
const matchedPrefixIndex = prefixMatch.matchedIndex
// ==========================================================================
// Disambiguation Timer (managed via phase state)
// ==========================================================================
// Timer ref for the disambiguation timeout
const disambiguationTimerRef = useRef<NodeJS.Timeout | null>(null)
// Manage disambiguation timer based on phase
useEffect(() => {
// Clear any existing timer
if (disambiguationTimerRef.current) {
clearTimeout(disambiguationTimerRef.current)
disambiguationTimerRef.current = null
}
// LINEAR parts never enter awaitingDisambiguation (see handleDigit); with the
// abacus scaffolding disabled there is no bead-help to schedule. Defensive.
if (disableAbacusScaffolding) return
// Only run timer when in awaitingDisambiguation phase
if (phase.phase !== 'awaitingDisambiguation') return
const ctx = phase.disambiguationContext
const elapsed = Date.now() - ctx.timerStartedAt
if (elapsed >= AMBIGUOUS_HELP_DELAY_MS) {
// Timer already elapsed - transition to help mode immediately
// Keep userAnswer during transition so it shows in answer boxes while fading out
const helpContext = computeHelpContext(phase.attempt.problem.terms, ctx.helpTermIndex)
setPhase({
phase: 'helpMode',
attempt: { ...phase.attempt, userAnswer: '' },
helpContext,
})
} else {
// Set timer for remaining time
const remaining = AMBIGUOUS_HELP_DELAY_MS - elapsed
disambiguationTimerRef.current = setTimeout(() => {
setPhase((prev) => {
if (prev.phase !== 'awaitingDisambiguation') return prev
const helpContext = computeHelpContext(
prev.attempt.problem.terms,
prev.disambiguationContext.helpTermIndex
)
return {
phase: 'helpMode',
attempt: { ...prev.attempt, userAnswer: '' },
helpContext,
}
})
}, remaining)
}
return () => {
if (disambiguationTimerRef.current) {
clearTimeout(disambiguationTimerRef.current)
}
}
}, [phase, disableAbacusScaffolding])
// Derive which term to show "need help?" prompt for from the phase
// Returns -1 when not in awaitingDisambiguation phase
const ambiguousHelpTermIndex = useMemo(() => {
if (phase.phase !== 'awaitingDisambiguation') return -1
return phase.disambiguationContext.helpTermIndex
}, [phase])
const canSubmit = useMemo(() => {
if (!attempt || !attempt.userAnswer) return false
const answerNum = parseInt(attempt.userAnswer, 10)
return !Number.isNaN(answerNum)
}, [attempt])
const shouldAutoSubmit = useMemo(() => {
// Don't auto-submit in awaitingDisambiguation - the input might be a prefix sum
if (phase.phase !== 'inputting' && phase.phase !== 'helpMode') return false
if (!attempt || !attempt.userAnswer) return false
if (attempt.correctionCount > MANUAL_SUBMIT_THRESHOLD) return false
const answerNum = parseInt(attempt.userAnswer, 10)
if (Number.isNaN(answerNum)) return false
return answerNum === attempt.problem.answer
}, [phase.phase, attempt])
// UI predicates
const canAcceptInput =
phase.phase === 'inputting' ||
phase.phase === 'awaitingDisambiguation' ||
phase.phase === 'helpMode'
const showAsCompleted = phase.phase === 'showingFeedback'
const showHelpOverlay = phase.phase === 'helpMode'
const showInputArea =
phase.phase === 'inputting' ||
phase.phase === 'awaitingDisambiguation' ||
phase.phase === 'helpMode' ||
phase.phase === 'submitting'
const showFeedback = phase.phase === 'showingFeedback' && phase.result === 'incorrect'
const inputIsFocused =
phase.phase === 'inputting' ||
phase.phase === 'awaitingDisambiguation' ||
phase.phase === 'helpMode'
const isTransitioning = phase.phase === 'transitioning'
const isPaused = phase.phase === 'paused'
const isSubmitting = phase.phase === 'submitting'
// Extract helpContext from phase
const helpContext = useMemo((): HelpContext | null => {
if (phase.phase === 'helpMode') {
return phase.helpContext
}
// Also check paused phase
if (phase.phase === 'paused' && phase.resumePhase.phase === 'helpMode') {
return phase.resumePhase.helpContext
}
return null
}, [phase])
// Extract outgoingAttempt from phase (only during transitions)
const outgoingAttempt = useMemo((): OutgoingAttempt | null => {
if (phase.phase === 'transitioning') {
return phase.outgoing
}
return null
}, [phase])
// ==========================================================================
// Actions
// ==========================================================================
const loadProblem = useCallback(
(problem: GeneratedProblem, slotIndex: number, partIndex: number) => {
const newAttempt = createAttemptInput(problem, slotIndex, partIndex)
setPhase({ phase: 'inputting', attempt: newAttempt })
},
[]
)
// ==========================================================================
// React to activeProblem changes (single source of truth)
// ==========================================================================
// This effect handles two cases:
// 1. activeProblem.key changes (redo mode, direct navigation, session plan advances)
// 2. Phase becomes 'loading' (need to reload current problem after incorrect answer or part transition)
const prevActiveProblemKeyRef = useRef<string | null>(null)
const hasMountedRef = useRef(false)
useEffect(() => {
// Skip on initial mount - let initialProblem handle hydration
if (!hasMountedRef.current) {
hasMountedRef.current = true
// Initialize the ref with current key
prevActiveProblemKeyRef.current = activeProblem?.key ?? null
return
}
// If no active problem, nothing to do
if (!activeProblem) {
prevActiveProblemKeyRef.current = null
return
}
const prevKey = prevActiveProblemKeyRef.current
const currentKey = activeProblem.key
const keyChanged = prevKey !== currentKey
// Case 1: Phase is 'loading' - need to load the problem
// This happens after incorrect answers or when returning from part transitions
if (phase.phase === 'loading') {
const newAttempt = createAttemptInput(
activeProblem.problem,
activeProblem.slotIndex,
activeProblem.partIndex
)
setPhase({ phase: 'inputting', attempt: newAttempt })
prevActiveProblemKeyRef.current = currentKey
return
}
// Case 2: Key changed - handle redo mode or session advancement
if (keyChanged) {
// CRITICAL: Don't interrupt normal progression flow
// If we're in showingFeedback, submitting, or transitioning, the normal flow
// (startTransition → completeTransition) handles advancing to the next problem.
const isInProgressionFlow =
phase.phase === 'showingFeedback' ||
phase.phase === 'submitting' ||
phase.phase === 'transitioning'
if (isInProgressionFlow) {
// Still update the ref so we track the change
prevActiveProblemKeyRef.current = currentKey
return
}
// Key changed and we're not in progression flow - load the new problem immediately
const newAttempt = createAttemptInput(
activeProblem.problem,
activeProblem.slotIndex,
activeProblem.partIndex
)
setPhase({ phase: 'inputting', attempt: newAttempt })
prevActiveProblemKeyRef.current = currentKey
}
}, [activeProblem, phase.phase])
const handleDigit = useCallback(
(digit: string) => {
setPhase((prev) => {
// Accept input in inputting, awaitingDisambiguation, or helpMode
if (
prev.phase !== 'inputting' &&
prev.phase !== 'awaitingDisambiguation' &&
prev.phase !== 'helpMode'
) {
return prev
}
// If in help mode, exit help mode and start fresh with the new digit
// We reset to an empty answer and fall through to the normal digit processing
// so that ambiguity checks (awaitingDisambiguation) still apply
if (prev.phase === 'helpMode') {
prev = {
phase: 'inputting',
attempt: { ...prev.attempt, userAnswer: '', rejectedDigit: null },
}
}
const attempt = prev.attempt
const sums = computePrefixSums(attempt.problem.terms)
// Once manual submit is required — or during LINEAR parts, where the abacus
// prefix-sum scaffolding is disabled — accept any digit without validation
// (never reject, never enter awaitingDisambiguation/help mode).
if (attempt.manualSubmitRequired || disableScaffoldingRef.current) {
const updatedAttempt = {
...attempt,
userAnswer: attempt.userAnswer + digit,
rejectedDigit: null,
}
return { phase: 'inputting', attempt: updatedAttempt }
}
if (isDigitConsistent(attempt.userAnswer, digit, sums)) {
const updatedAttempt = {
...attempt,
userAnswer: attempt.userAnswer + digit,
rejectedDigit: null,
}
// Check if the new answer creates an ambiguous prefix match
const newPrefixMatch = findMatchedPrefixIndex(updatedAttempt.userAnswer, sums)
if (newPrefixMatch.isAmbiguous && newPrefixMatch.helpTermIndex >= 0) {
// Ambiguous match - transition to awaitingDisambiguation
return {
phase: 'awaitingDisambiguation',
attempt: updatedAttempt,
disambiguationContext: {
matchedPrefixIndex: newPrefixMatch.matchedIndex,
helpTermIndex: newPrefixMatch.helpTermIndex,
timerStartedAt: Date.now(),
},
}
} else if (
!newPrefixMatch.isAmbiguous &&
newPrefixMatch.matchedIndex >= 0 &&
newPrefixMatch.helpTermIndex >= 0
) {
// Unambiguous intermediate prefix match (e.g., "03" for prefix sum 3)
// Immediately enter help mode
// Keep userAnswer during transition so it shows in answer boxes while fading out
const helpContext = computeHelpContext(
attempt.problem.terms,
newPrefixMatch.helpTermIndex
)
return {
phase: 'helpMode',
attempt: { ...updatedAttempt, userAnswer: '' },
helpContext,
}
} else {
// No special prefix match - stay in inputting (or return to it from awaitingDisambiguation)
return { phase: 'inputting', attempt: updatedAttempt }
}
} else {
// Reject the digit
const newCorrectionCount = attempt.correctionCount + 1
const nowRequiresManualSubmit =
newCorrectionCount > MANUAL_SUBMIT_THRESHOLD && !attempt.manualSubmitRequired
if (nowRequiresManualSubmit) {
setTimeout(() => onManualSubmitRequired?.(), 0)
}
const updatedAttempt = {
...attempt,
rejectedDigit: digit,
correctionCount: newCorrectionCount,
manualSubmitRequired: attempt.manualSubmitRequired || nowRequiresManualSubmit,
}
// Keep the same phase type but update attempt
if (prev.phase === 'awaitingDisambiguation') {
return { ...prev, attempt: updatedAttempt }
}
return { ...prev, attempt: updatedAttempt }
}
})
// Clear rejected digit after animation
setTimeout(() => {
setPhase((prev) => {
if (
prev.phase !== 'inputting' &&
prev.phase !== 'awaitingDisambiguation' &&
prev.phase !== 'helpMode'
) {
return prev
}
return { ...prev, attempt: { ...prev.attempt, rejectedDigit: null } }
})
}, 300)
},
[onManualSubmitRequired]
)
const handleBackspace = useCallback(() => {
setPhase((prev) => {
if (
prev.phase !== 'inputting' &&
prev.phase !== 'awaitingDisambiguation' &&
prev.phase !== 'helpMode'
) {
return prev
}
const attempt = prev.attempt
if (attempt.userAnswer.length === 0) return prev
const newCorrectionCount = attempt.correctionCount + 1
const nowRequiresManualSubmit =
newCorrectionCount > MANUAL_SUBMIT_THRESHOLD && !attempt.manualSubmitRequired
if (nowRequiresManualSubmit) {
setTimeout(() => onManualSubmitRequired?.(), 0)
}
const updatedAttempt = {
...attempt,
userAnswer: attempt.userAnswer.slice(0, -1),
correctionCount: newCorrectionCount,
manualSubmitRequired: attempt.manualSubmitRequired || nowRequiresManualSubmit,
}
// After backspace, always return to inputting phase (no longer ambiguous)
return { phase: 'inputting', attempt: updatedAttempt }
})
}, [onManualSubmitRequired])
const enterHelpMode = useCallback((termIndex: number) => {
// LINEAR parts have the abacus put away — bead-decomposition help must never
// be entered. No-op keeps the current phase and typed answer intact.
if (disableScaffoldingRef.current) return
setPhase((prev) => {
// Allow entering help mode from inputting, awaitingDisambiguation, or helpMode (to navigate to a different term)
if (
prev.phase !== 'inputting' &&
prev.phase !== 'awaitingDisambiguation' &&
prev.phase !== 'helpMode'
) {
return prev
}
// Keep userAnswer during transition so it shows in answer boxes while fading out
const helpContext = computeHelpContext(prev.attempt.problem.terms, termIndex)
return {
phase: 'helpMode',
attempt: { ...prev.attempt, userAnswer: '' },
helpContext,
}
})
}, [])
const exitHelpMode = useCallback(() => {
setPhase((prev) => {
if (prev.phase === 'helpMode') {
const updatedAttempt = { ...prev.attempt, userAnswer: '' }
return { phase: 'inputting', attempt: updatedAttempt }
}
if (prev.phase === 'inputting') {
const updatedAttempt = { ...prev.attempt, userAnswer: '' }
return { phase: 'inputting', attempt: updatedAttempt }
}
return prev
})
}, [])
const clearAnswer = useCallback(() => {
setPhase((prev) => {
if (prev.phase !== 'helpMode') return prev
const updatedAttempt = { ...prev.attempt, userAnswer: '' }
return {
phase: 'helpMode',
attempt: updatedAttempt,
helpContext: prev.helpContext,
}
})
}, [])
const setAnswer = useCallback((value: string) => {
setPhase((prev) => {
if (prev.phase !== 'helpMode' && prev.phase !== 'inputting') return prev
const updatedAttempt = { ...prev.attempt, userAnswer: value }
if (prev.phase === 'helpMode') {
return {
phase: 'helpMode',
attempt: updatedAttempt,
helpContext: prev.helpContext,
}
}
return { phase: 'inputting', attempt: updatedAttempt }
})
}, [])
const startSubmit = useCallback(() => {
setPhase((prev) => {
// Allow submitting from inputting, awaitingDisambiguation, or helpMode
if (
prev.phase !== 'inputting' &&
prev.phase !== 'awaitingDisambiguation' &&
prev.phase !== 'helpMode'
) {
return prev
}
return { phase: 'submitting', attempt: prev.attempt }
})
}, [])
const completeSubmit = useCallback((result: 'correct' | 'incorrect') => {
setPhase((prev) =>
transformActivePhase(prev, (active) => {
if (active.phase !== 'submitting') return null
return { phase: 'showingFeedback', attempt: active.attempt, result }
})
)
}, [])
const startTransition = useCallback(
(nextProblem: GeneratedProblem, nextSlotIndex: number, nextPartIndex?: number) => {
setPhase((prev) => {
if (prev.phase !== 'showingFeedback') return prev
const outgoing: OutgoingAttempt = {
key: `${prev.attempt.partIndex}-${prev.attempt.slotIndex}`,
problem: prev.attempt.problem,
userAnswer: prev.attempt.userAnswer,
result: prev.result,
partIndex: prev.attempt.partIndex,
}
const incomingPartIndex = nextPartIndex ?? prev.attempt.partIndex
const incoming = createAttemptInput(nextProblem, nextSlotIndex, incomingPartIndex)
return { phase: 'transitioning', outgoing, incoming }
})
},
[]
)
const completeTransition = useCallback(() => {
setPhase((prev) =>
transformActivePhase(prev, (active) => {
if (active.phase !== 'transitioning') return null
return { phase: 'inputting', attempt: active.incoming }
})
)
}, [])
const clearToLoading = useCallback(() => {
setPhase({ phase: 'loading' })
}, [])
const markComplete = useCallback(() => {
setPhase({ phase: 'complete' })
}, [])
const pause = useCallback(() => {
// Capture the hidden clock BEFORE the (pure) updater so StrictMode's
// double-invocation can't fold the same span twice.
const now = Date.now()
const hiddenAt = hiddenAtRef.current
setPhase((prev) => {
if (prev.phase === 'paused' || prev.phase === 'loading' || prev.phase === 'complete')
return prev
// Disjoint-union rule (b): a hidden span in flight is folded into the
// attempt FIRST (clamped to its lifetime); from here the pause clock owns
// elapsed time, so the two never double-count.
const base = hiddenAt != null ? foldHiddenSpanIntoActivePhase(prev, hiddenAt, now) : prev
return { phase: 'paused', resumePhase: base, pauseStartedAt: now }
})
// The hidden span (if any) has been handed off to the pause clock.
hiddenAtRef.current = null
}, [])
const resume = useCallback(() => {
const now = Date.now()
// Disjoint-union rule (c): if the tab is STILL hidden (teacher remote-resume
// while backgrounded), re-arm the hidden clock so the still-hidden span
// keeps being excluded when the child actually returns.
const stillHidden = typeof document !== 'undefined' && document.visibilityState === 'hidden'
setPhase((prev) => {
if (prev.phase !== 'paused') return prev
// Fold the pause span into the attempt via the shared accounting.
return addPauseToActivePhase(prev.resumePhase, now - prev.pauseStartedAt, { hidden: false })
})
hiddenAtRef.current = stillHidden ? now : null
}, [])
// ==========================================================================
// Visibility layer (#156): fold hidden-tab time out of the response time
// ==========================================================================
//
// Mount-only. On hidden we arm `hiddenAtRef`; on return we fold the elapsed
// span into the current attempt (silently — no phase change, so no pause
// modal flashes). `visibilitychange` is primary; `pagehide` / persisted
// `pageshow` are idempotent belt-and-suspenders for iOS Safari reliability.
useEffect(() => {
if (typeof document === 'undefined') return
const arm = () => {
// Rule (a): arm only when not already armed (idempotent under duplicate
// pagehide + visibilitychange) and not paused (the pause clock owns it).
if (hiddenAtRef.current != null) return
if (phaseRef.current.phase === 'paused') return
hiddenAtRef.current = Date.now()
}
const fold = () => {
const hiddenAt = hiddenAtRef.current
if (hiddenAt == null) return
hiddenAtRef.current = null
const now = Date.now()
setPhase((prev) => {
// Rule (d): visible while paused → no-op (pause clock owns the interval).
if (prev.phase === 'paused') return prev
return foldHiddenSpanIntoActivePhase(prev, hiddenAt, now)
})
}
const handleVisibilityChange = () => {
if (document.visibilityState === 'hidden') arm()
else fold()
}
const handlePageHide = () => arm()
const handlePageShow = (event: PageTransitionEvent) => {
if (event.persisted) fold()
}
document.addEventListener('visibilitychange', handleVisibilityChange)
window.addEventListener('pagehide', handlePageHide)
window.addEventListener('pageshow', handlePageShow)
return () => {
document.removeEventListener('visibilitychange', handleVisibilityChange)
window.removeEventListener('pagehide', handlePageHide)
window.removeEventListener('pageshow', handlePageShow)
}
}, [])
// Is the session complete?
const isComplete = phase.phase === 'complete'
return {
phase,
attempt,
helpContext,
outgoingAttempt,
canAcceptInput,
showAsCompleted,
showHelpOverlay,
showInputArea,
showFeedback,
inputIsFocused,
isTransitioning,
isPaused,
isSubmitting,
prefixSums,
prefixMatch,
matchedPrefixIndex,
canSubmit,
shouldAutoSubmit,
ambiguousHelpTermIndex,
loadProblem,
handleDigit,
handleBackspace,
enterHelpMode,
exitHelpMode,
clearAnswer,
setAnswer,
startSubmit,
completeSubmit,
startTransition,
completeTransition,
clearToLoading,
markComplete,
pause,
resume,
isComplete,
}
}
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