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* Constraint Parser
*
* Parses correctAnswer expressions from flowchart decision nodes into
* structured constraints that can guide problem generation.
*
* Recognizes patterns like:
* - "a == b" (equality)
* - "a != b" (inequality)
* - "a > b", "a >= b", "a < b", "a <= b" (comparison)
* - "a % b == 0" (divisibility)
* - "a == 0", "a != 0" (zero checks)
* - Boolean expressions with && and ||
*/
// =============================================================================
// Types
// =============================================================================
export type ComparisonOp = '==' | '!=' | '>' | '>=' | '<' | '<='
/**
* A parsed constraint representing a comparison between two operands.
*/
export interface ParsedConstraint {
/** Type of constraint */
type: 'comparison' | 'divisibility' | 'boolean'
/** Left operand (field name or expression) */
left: string
/** Comparison operator */
op: ComparisonOp
/** Right operand (field name, literal, or expression) */
right: string
/**
* Whether the right side is a literal value.
* If true, we can directly constrain the left field.
*/
rightIsLiteral: boolean
/**
* The literal value if rightIsLiteral is true.
*/
literalValue?: number | string | boolean
/**
* For divisibility constraints (a % b == 0), the divisor field.
*/
divisor?: string
/**
* The original expression string.
*/
original: string
}
/**
* Result of parsing a correctAnswer expression.
*/
export interface ParseResult {
/** Successfully parsed constraints */
constraints: ParsedConstraint[]
/** Whether the expression was fully parsed */
fullyParsed: boolean
/** Original expression */
original: string
}
// =============================================================================
// Parser
// =============================================================================
/**
* Parse a correctAnswer expression into structured constraints.
*
* @param expression The correctAnswer expression string
* @returns ParseResult with extracted constraints
*/
export function parseConstraintExpression(expression: string): ParseResult {
const constraints: ParsedConstraint[] = []
const original = expression.trim()
// Handle simple boolean literals
if (original === 'true' || original === 'false') {
return {
constraints: [
{
type: 'boolean',
left: original,
op: '==',
right: 'true',
rightIsLiteral: true,
literalValue: original === 'true',
original,
},
],
fullyParsed: true,
original,
}
}
// Try to parse as a single comparison
const singleConstraint = parseSingleComparison(original)
if (singleConstraint) {
constraints.push(singleConstraint)
return { constraints, fullyParsed: true, original }
}
// Try to parse && expressions (all must be true)
if (original.includes('&&')) {
const parts = splitOnOperator(original, '&&')
let allParsed = true
for (const part of parts) {
const constraint = parseSingleComparison(part.trim())
if (constraint) {
constraints.push(constraint)
} else {
allParsed = false
}
}
return { constraints, fullyParsed: allParsed, original }
}
// Try to parse || expressions (any can be true)
// For generation, we'll pick one branch to satisfy
if (original.includes('||')) {
const parts = splitOnOperator(original, '||')
// For OR expressions, we parse all branches but mark as not fully parsed
// since the generator needs to decide which branch to take
for (const part of parts) {
const constraint = parseSingleComparison(part.trim())
if (constraint) {
constraints.push(constraint)
}
}
return { constraints, fullyParsed: false, original }
}
// Couldn't parse - return empty constraints
return { constraints: [], fullyParsed: false, original }
}
/**
* Parse a single comparison expression like "a == b" or "a != 0".
*/
function parseSingleComparison(expr: string): ParsedConstraint | null {
const trimmed = expr.trim()
// Remove outer parentheses if present
const unwrapped = unwrapParens(trimmed)
// Check for divisibility pattern: a % b == 0
const divMatch = unwrapped.match(/^(.+?)\s*%\s*(.+?)\s*==\s*0$/)
if (divMatch) {
return {
type: 'divisibility',
left: divMatch[1].trim(),
op: '==',
right: '0',
rightIsLiteral: true,
literalValue: 0,
divisor: divMatch[2].trim(),
original: expr,
}
}
// Check for comparison operators (order matters - check >= before >)
const ops: ComparisonOp[] = ['==', '!=', '>=', '<=', '>', '<']
for (const op of ops) {
const parts = splitOnOperator(unwrapped, op)
if (parts.length === 2) {
const left = parts[0].trim()
const right = parts[1].trim()
// Check if right side is a literal
const literalValue = parseLiteral(right)
const rightIsLiteral = literalValue !== undefined
return {
type: 'comparison',
left,
op,
right,
rightIsLiteral,
literalValue,
original: expr,
}
}
}
return null
}
/**
* Split an expression on an operator, respecting parentheses.
*/
function splitOnOperator(expr: string, operator: string): string[] {
const result: string[] = []
let current = ''
let parenDepth = 0
let i = 0
while (i < expr.length) {
const char = expr[i]
if (char === '(') {
parenDepth++
current += char
i++
} else if (char === ')') {
parenDepth--
current += char
i++
} else if (parenDepth === 0 && expr.substring(i, i + operator.length) === operator) {
result.push(current)
current = ''
i += operator.length
} else {
current += char
i++
}
}
if (current) {
result.push(current)
}
return result
}
/**
* Remove outer parentheses from an expression.
*/
function unwrapParens(expr: string): string {
const trimmed = expr.trim()
if (trimmed.startsWith('(') && trimmed.endsWith(')')) {
// Check if the parens actually wrap the whole expression
let depth = 0
for (let i = 0; i < trimmed.length - 1; i++) {
if (trimmed[i] === '(') depth++
if (trimmed[i] === ')') depth--
if (depth === 0) return trimmed // Parens don't wrap whole thing
}
return unwrapParens(trimmed.slice(1, -1))
}
return trimmed
}
/**
* Try to parse a string as a literal value.
*/
function parseLiteral(value: string): number | string | boolean | undefined {
const trimmed = value.trim()
// Boolean
if (trimmed === 'true') return true
if (trimmed === 'false') return false
// Number
const num = parseFloat(trimmed)
if (!isNaN(num) && isFinite(num) && String(num) === trimmed) {
return num
}
// Integer (handles cases like "0" that parseFloat would also handle)
const int = parseInt(trimmed, 10)
if (!isNaN(int) && String(int) === trimmed) {
return int
}
// String literal (quoted)
if (
(trimmed.startsWith("'") && trimmed.endsWith("'")) ||
(trimmed.startsWith('"') && trimmed.endsWith('"'))
) {
return trimmed.slice(1, -1)
}
return undefined
}
// =============================================================================
// Constraint Analysis
// =============================================================================
/**
* Extract field names referenced in a constraint.
*/
export function extractFieldsFromConstraint(constraint: ParsedConstraint): string[] {
const fields: string[] = []
// Add left side (could be a field or expression)
const leftFields = extractFieldsFromExpression(constraint.left)
fields.push(...leftFields)
// Add right side if not a literal
if (!constraint.rightIsLiteral) {
const rightFields = extractFieldsFromExpression(constraint.right)
fields.push(...rightFields)
}
// Add divisor if present
if (constraint.divisor) {
const divisorFields = extractFieldsFromExpression(constraint.divisor)
fields.push(...divisorFields)
}
return [...new Set(fields)] // Deduplicate
}
/**
* Extract field names from an expression string.
* Recognizes patterns like: fieldName, obj.field, field1 + field2
*/
function extractFieldsFromExpression(expr: string): string[] {
const fields: string[] = []
// Match identifiers (including dot notation like left.denom)
const identifierRegex = /[a-zA-Z_][a-zA-Z0-9_]*(?:\.[a-zA-Z_][a-zA-Z0-9_]*)*/g
const matches = expr.match(identifierRegex) || []
for (const match of matches) {
// Filter out keywords and function names
const keywords = [
'true',
'false',
'null',
'undefined',
'floor',
'ceil',
'abs',
'min',
'max',
'lcm',
'gcd',
]
if (!keywords.includes(match)) {
fields.push(match)
}
}
return fields
}
/**
* Negate a constraint (for when correctAnswer must be false).
*/
export function negateConstraint(constraint: ParsedConstraint): ParsedConstraint {
const negatedOps: Record<ComparisonOp, ComparisonOp> = {
'==': '!=',
'!=': '==',
'>': '<=',
'>=': '<',
'<': '>=',
'<=': '>',
}
return {
...constraint,
op: negatedOps[constraint.op],
original: `!(${constraint.original})`,
}
}
/**
* Determine if a constraint can be directly used to filter field values.
* Returns the field name and filter function if applicable.
*/
export interface FieldFilter {
field: string
filter: (value: number | string) => boolean
description: string
}
export function constraintToFieldFilter(constraint: ParsedConstraint): FieldFilter | null {
// Only handle simple cases where left is a single field and right is a literal
if (!constraint.rightIsLiteral) return null
if (constraint.literalValue === undefined) return null
const field = constraint.left
const literal = constraint.literalValue
// Check if left is a simple field (no operators)
if (field.includes('+') || field.includes('-') || field.includes('*') || field.includes('/')) {
return null
}
switch (constraint.op) {
case '==':
return {
field,
filter: (v) => v === literal,
description: `${field} must equal ${literal}`,
}
case '!=':
return {
field,
filter: (v) => v !== literal,
description: `${field} must not equal ${literal}`,
}
case '>':
if (typeof literal === 'number') {
return {
field,
filter: (v) => typeof v === 'number' && v > literal,
description: `${field} must be greater than ${literal}`,
}
}
break
case '>=':
if (typeof literal === 'number') {
return {
field,
filter: (v) => typeof v === 'number' && v >= literal,
description: `${field} must be at least ${literal}`,
}
}
break
case '<':
if (typeof literal === 'number') {
return {
field,
filter: (v) => typeof v === 'number' && v < literal,
description: `${field} must be less than ${literal}`,
}
}
break
case '<=':
if (typeof literal === 'number') {
return {
field,
filter: (v) => typeof v === 'number' && v <= literal,
description: `${field} must be at most ${literal}`,
}
}
break
}
return null
}
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