package support;

import java.util.regex.Pattern;
import java.util.regex.PatternSyntaxException;

/**
 * MOAD-0011: CWE-1333 — A Catastrophic Inheritance.
 *
 * Defect: a PCRE-compatible regex engine inherits Perl's backtracking NFA
 * semantics.  Applied to user-controlled input with nested quantifiers or
 * ambiguous alternation, the engine explores O(2^N) parse trees before
 * concluding no match.  The defect propagates through intellectual lineage:
 * Perl designed the backtracking NFA in 1987, PCRE codified it, and every
 * language that adopted Perl's regex syntax inherited the worst-case complexity.
 *
 * Confirmed instance: bleach 3.x sanitize_css gauntlet pattern — 12-second hang
 * on 71-char input.  Java's java.util.regex uses backtracking NFA and exhibits
 * the same catastrophic behavior on the same class of patterns.
 *
 * Fix: restructure the pattern to eliminate nested quantifiers and ambiguous
 * alternation, or switch to a linear NFA engine (RE2, Rust regex) that
 * guarantees O(N) matching with no catastrophic backtracking.
 *
 * This specimen proves the defect structurally (nested quantifier detection)
 * and behaviorally at safe scale (step counting on a small custom NFA).
 *
 * Scanner detects: Pattern.compile / re.compile / RegExp constructor called with
 * patterns containing nested quantifiers: (X+)+, (X*)+, (X+)*, (X|Y+)+, etc.
 */
public class Moad0011Algorithm {

    // ── Structural detection ──────────────────────────────────────────────────

    /**
     * Returns true if the regex pattern string contains nested quantifiers —
     * the structural signature of catastrophic backtracking risk.
     *
     * Detects patterns of the form (group-with-quantifier) followed by +, *, ?, {n,}.
     * Examples that match: (a+)+  (a*)+  (a+b*)+  (a+)+?  (a+){2,}
     * Examples that do not: a+  (a+)  (a|b)+  [a-z]+
     */
    public static boolean hasCatastrophicStructure(String pattern) {
        // Regex to detect nested quantifiers:
        // A group whose contents contain a quantifier, followed by an outer quantifier.
        // Inner: \([^)]*[+*?][^)]*\) — a paren group containing +, *, or ?
        // Outer: [+*?] or {\d+,?\d*} following the closing paren
        return pattern.matches(".*\\([^)]*[+*?][^)]*\\)[+*?{].*");
    }

    /**
     * Returns true if the pattern is safe: no nested quantifiers detected.
     */
    public static boolean isSafePattern(String pattern) {
        return !hasCatastrophicStructure(pattern);
    }

    // ── Known defective and fixed pattern pairs ───────────────────────────────

    /**
     * Defective: (a+)+b — nested quantifier.
     * The outer + allows grouping any prefix of a's; the inner + requires at
     * least one.  On input "aaa...c" (no 'b'), the engine explores all ways to
     * partition the a-run into groups — O(2^N) attempts.
     */
    public static final String DEFECTIVE_PATTERN_BLEACH = "(a+)+b";

    /**
     * Fixed: a+b — equivalent semantics, no nesting.
     * Matches one or more a's followed by b.  On "aaa...c" it fails in O(N).
     */
    public static final String FIXED_PATTERN_BLEACH = "a+b";

    /**
     * Defective: CSS-style gauntlet (simplified from bleach 3.x).
     * Ambiguous alternation with nested quantifiers across CSS property branches.
     */
    public static final String DEFECTIVE_PATTERN_CSS =
        "([a-z0-9]+[\\s]*:[\\s]*(([a-z0-9]+[\\s]*)+(,|;|$))+)";

    /**
     * Fixed: anchored, non-ambiguous CSS value pattern.
     */
    public static final String FIXED_PATTERN_CSS =
        "^[a-z0-9][a-z0-9\\s]*:[a-z0-9,;\\s]+$";

    // ── Behavioral proof: step-counting NFA for (a+)+b ───────────────────────

    /**
     * Result of a regex match attempt: whether it matched and how many NFA
     * steps were taken.  Step count proves exponential vs linear growth.
     */
    public static final class Result {
        public final boolean matched;
        public final long    steps;

        public Result(boolean matched, long steps) {
            this.matched = matched;
            this.steps   = steps;
        }
    }

    /**
     * Defective NFA: matches (a+)+b with explicit backtracking step counting.
     *
     * Implements the nested quantifier pattern by trying all possible
     * partitions of a leading run of 'a' characters.  On non-matching input
     * (no trailing 'b'), explores O(2^N) states.
     *
     * @param input the string to match
     * @return Result with matched=true iff input matches, steps=NFA steps taken
     */
    public static Result matchDefective(String input) {
        long[] stepCounter = {0L};
        boolean matched = defectiveMatch(input, 0, stepCounter);
        return new Result(matched, stepCounter[0]);
    }

    /**
     * Recursive backtracking NFA for (a+)+b.
     *
     * State: consumed 'pos' characters of input.  Outer quantifier tries at
     * least one inner (a+) group.  Each inner group consumes 1..k a's.  The
     * outer loop repeats until no more a's can be consumed, then checks for 'b'.
     */
    private static boolean defectiveMatch(String input, int pos, long[] steps) {
        steps[0]++;
        return outerPlus(input, pos, steps);
    }

    /** Outer (...)+ — must match at least one inner group. */
    private static boolean outerPlus(String input, int pos, long[] steps) {
        // Try consuming one inner (a+) group starting at 'pos', then recurse outer.
        return innerPlus(input, pos, steps, true);
    }

    /**
     * Inner a+ — consume 1..k a's, then try continuing the outer loop or matching 'b'.
     *
     * @param first true if this is the first attempt at the outer loop (must match at least once)
     */
    private static boolean innerPlus(String input, int pos, long[] steps, boolean first) {
        steps[0]++;
        if (pos >= input.length() || input.charAt(pos) != 'a') {
            // Cannot consume an 'a' — try ending outer loop and matching 'b'
            if (!first) {
                return matchB(input, pos, steps);
            }
            return false; // outer + needs at least one inner group
        }

        // Greedy: try consuming as many a's as possible, then backtrack
        int start = pos;
        // Count maximum a's from here
        int maxPos = pos;
        while (maxPos < input.length() && input.charAt(maxPos) == 'a') maxPos++;

        // Try splitting: consume (maxPos - start) down to 1 a's in inner group,
        // then continue outer loop or end
        for (int endInner = maxPos; endInner > start; endInner--) {
            steps[0]++;
            // After consuming [start..endInner) a's, try continuing outer loop
            if (innerPlus(input, endInner, steps, false)) return true;
        }
        return false;
    }

    /** Match literal 'b' at position pos. */
    private static boolean matchB(String input, int pos, long[] steps) {
        steps[0]++;
        return pos < input.length() && input.charAt(pos) == 'b' && pos + 1 == input.length();
    }

    /**
     * Fixed NFA: matches a+b in O(N) steps — no backtracking, no nested quantifiers.
     *
     * @param input the string to match
     * @return Result with matched=true iff input matches a+b exactly, steps=O(N)
     */
    public static Result matchFixed(String input) {
        long steps = 0;
        int pos = 0;

        // Must have at least one 'a'
        if (pos >= input.length() || input.charAt(pos) != 'a') {
            return new Result(false, ++steps);
        }

        // Consume all leading a's — O(N) scan, single pass, no backtracking
        while (pos < input.length() && input.charAt(pos) == 'a') {
            pos++;
            steps++;
        }

        // Must end with exactly one 'b' and nothing after
        steps++;
        boolean matched = (pos < input.length() && input.charAt(pos) == 'b'
                           && pos + 1 == input.length());
        return new Result(matched, steps);
    }

    /**
     * Build an adversarial non-matching input: N a's followed by 'c'.
     * Defective engine explores O(2^N) states; fixed engine scans O(N) and stops.
     */
    public static String adversarialInput(int aCount) {
        return "a".repeat(aCount) + "c";
    }

    /**
     * Build a matching input: N a's followed by 'b'.
     * Both engines return matched=true; step counts differ in constant factor.
     */
    public static String matchingInput(int aCount) {
        return "a".repeat(aCount) + "b";
    }
}