import Foundation
@testable import PureYAML
import Testing

/// Differential proof for the public `PureYAML.Streaming.Scanner`, the thin
/// translation wrapper around the internal `LibYAML.Scanner.scanStep()`
/// already proven by `StreamingScannerDifferentialTests`. That existing proof
/// covers `scanStep()` itself; this one covers the translation layer on top
/// of it (`Streaming.Token`/`Streaming.Token.Kind`/`Streaming.Token.ScalarStyle`
/// mapped from the internal `LibYAML.Token`/`TokenType`/`ScalarStyle`), since a
/// wrong case mapping or a dropped field would not be caught by a proof that
/// never looks at the public types at all.
@Suite("Public streaming scanner")
struct PublicStreamingScannerTests {
    @Test("Public Scanner matches the internal scanAll() reference, translated, 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",
        "&a a: b\nc: &d d\n",
    ])
    func test_publicScannerMatchesTranslatedReference(yaml: String) throws {
        try assertPublicScannerMatchesReference(yaml)
    }

    @Test("Public Scanner matches the internal scanAll() reference over the whole yaml-test-suite corpus")
    func test_publicScannerMatchesReferenceOverWholeSuite() 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 assertPublicScannerMatchesReference(yaml)
            } catch {
                mismatches.append("\(testCase.id): \(error)")
            }
        }
        #expect(mismatches.isEmpty, "public/internal mismatches (\(mismatches.count)): \(mismatches.prefix(10).joined(separator: "; "))")
    }
}

private func assertPublicScannerMatchesReference(_ yaml: String) throws {
    let reference = translatedBatchReference(yaml)
    for chunkSize in [1, 2, 3, 7, 64, yaml.unicodeScalars.count + 1] {
        let observed = try publicScan(yaml, chunkSize: chunkSize)
        #expect(observed == reference, "chunk size \(chunkSize) diverged from the translated reference for: \(yaml.debugDescription)")
    }
}

private enum PublicScanResult: Equatable {
    case tokens([PureYAML.Streaming.Token])
    case threw(String)
}

/// The reference: the batch scanner's own tokens, run through the exact same
/// `Streaming.Token.init(_:)` translator the public `Scanner` uses, so this
/// proof isolates "does the public wrapper preserve what the internal scanner
/// already produces" from "is the internal scanner itself correct" (the
/// latter is `StreamingScannerDifferentialTests`'s job, not this one's).
private func translatedBatchReference(_ yaml: String) -> PublicScanResult {
    var scanner = PureYAML.Parsing.LibYAML.Scanner(yaml)
    do {
        return try .tokens(scanner.scanAll().map(PureYAML.Streaming.Token.init))
    } catch {
        return .threw("\(error)")
    }
}

private func publicScan(_ yaml: String, chunkSize: Int) throws -> PublicScanResult {
    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.Streaming.Scanner()
    var nextChunk = 0
    var tokens: [PureYAML.Streaming.Token] = []
    do {
        while true {
            switch try scanner.next() {
            case let .token(token):
                tokens.append(token)
            case .done:
                return .tokens(tokens)
            case .needMoreInput:
                if nextChunk < chunks.count {
                    scanner.feed(chunks[nextChunk])
                    nextChunk += 1
                } else {
                    scanner.finish()
                }
            }
        }
    } catch {
        return .threw("\(error)")
    }
}