import Foundation
@testable import PureYAML
import Testing

/// Differential proof for the resumable `Scanner.scanStep()` (issue #4's first
/// increment: a checkpoint-and-retry wrapper around the existing batch
/// `fetchNextToken()`, see `LibYAML.Scanner.Streaming.swift`). The scanner is a
/// pure value type, so `scanStep()` reuses every fetch function unmodified; the
/// only new behavior is (1) `Reader` flags `ranDry` instead of asserting end of
/// input when `!isFinal`, and (2) tokens are released one at a time only when no
/// possible simple key still targets the queue front. Neither is exercised by
/// `scanAll()`, so this is genuinely new code path, not a refactor, and needs its
/// own proof: driving the same input through `scanStep()` at several chunk sizes
/// must reproduce `scanAll()`'s token array (or thrown error) exactly.
@Suite("Streaming scanner differential")
struct StreamingScannerDifferentialTests {
    /// A handful of representative always-on cases so `swift test` gets real
    /// coverage without the opt-in whole-suite flag: a plain scalar split
    /// mid-token, a block mapping, a flow collection, an anchor/alias pair, and
    /// a deliberately malformed input (an unterminated quoted scalar).
    @Test("Representative cases match scanAll() at several chunk sizes", arguments: [
        "title: Example API",
        """
        title: Example
        servers:
          - url: /
        """,
        "flow: {a: 1, b: [2, 3]}",
        """
        base: &anchor
          value: 1
        derived: *anchor
        """,
        "block: |\n  line one\n  line two\n",
        "title: \"unterminated",
        "&a a: b\nc: &d d\n",
    ])
    func test_representativeCasesMatchScanAllAtSeveralChunkSizes(yaml: String) throws {
        try assertStreamingMatchesBatch(yaml)
    }

    @Test("Every yaml-test-suite case matches scanAll() at several chunk sizes")
    func test_wholeSuiteMatchesScanAllAtSeveralChunkSizes() throws {
        guard ProcessInfo.processInfo.environment["PUREYAML_RUN_YAML_TEST_SUITE"] == "1" else {
            return
        }
        let manifest = try YAMLTestSuiteManifest.load()
        var mismatches: [String] = []
        for testCase in manifest.cases {
            let yaml = try testCase.loadInput()
            do {
                try assertStreamingMatchesBatch(yaml)
            } catch {
                mismatches.append("\(testCase.id): \(error)")
            }
        }
        #expect(mismatches.isEmpty, "streaming/batch mismatches (\(mismatches.count)): \(mismatches.prefix(10).joined(separator: "; "))")
    }
}

/// Runs the batch scanner once as the reference, then the streaming scanner at
/// several chunk sizes (including feeding the whole input as a single "chunk",
/// which should degenerate to one dry run followed by success), and asserts each
/// streaming run reproduces the reference token array or thrown error exactly.
private func assertStreamingMatchesBatch(_ yaml: String) throws {
    let reference = batchScan(yaml)
    for chunkSize in [1, 2, 3, 7, 64, yaml.unicodeScalars.count + 1] {
        let observed = try streamingScan(yaml, chunkSize: chunkSize)
        #expect(observed == reference, "chunk size \(chunkSize) diverged from scanAll() for: \(yaml.debugDescription)")
    }
}

private enum ScanResult: Equatable {
    case tokens([PureYAML.Parsing.LibYAML.Token])
    case threw(String)
}

private func batchScan(_ yaml: String) -> ScanResult {
    var scanner = PureYAML.Parsing.LibYAML.Scanner(yaml)
    do {
        return try .tokens(scanner.scanAll())
    } catch {
        return .threw("\(error)")
    }
}

/// Drives `scanStep()` over `yaml` split into `chunkSize`-scalar pieces, feeding
/// one chunk whenever the scanner reports `.needMoreInput`, and marking the
/// reader final once every chunk has been appended.
private func streamingScan(_ yaml: String, chunkSize: Int) throws -> ScanResult {
    let scalars = Array(yaml.unicodeScalars)
    var chunks: [String] = []
    var index = 0
    while index < scalars.count {
        let end = min(index + chunkSize, scalars.count)
        chunks.append(String(String.UnicodeScalarView(scalars[index ..< end])))
        index = end
    }

    var scanner = PureYAML.Parsing.LibYAML.Scanner("", isFinal: false)
    var nextChunk = 0
    var tokens: [PureYAML.Parsing.LibYAML.Token] = []
    do {
        while true {
            switch try scanner.scanStep() {
            case let .token(token):
                tokens.append(token)
            case .done:
                return .tokens(tokens)
            case .needMoreInput:
                if nextChunk < chunks.count {
                    scanner.reader.append(chunks[nextChunk])
                    nextChunk += 1
                } else {
                    scanner.reader.markFinal()
                }
            }
        }
    } catch {
        return .threw("\(error)")
    }
}