import Foundation

public struct GitDiffHunk: Identifiable, Sendable, Equatable {
    public let id: Int
    public let fileHeader: String
    public let lines: [String]
    public var patch: String { fileHeader + lines.joined(separator: "\n") + "\n" }
    public var title: String { lines.first ?? "Changes" }
}

public enum GitDiffParser {
    /// Retains each file's complete header so a selected hunk remains a valid Git patch.
    public static func hunks(_ diff: String) -> [GitDiffHunk] {
        var result: [GitDiffHunk] = []
        var header: [String] = []
        var lines: [String] = []
        func flush() {
            guard !lines.isEmpty else { return }
            result.append(
                GitDiffHunk(id: result.count, fileHeader: header.joined(separator: "\n") + "\n", lines: lines))
            lines.removeAll()
        }
        var source = diff.components(separatedBy: "\n")
        if source.last == "" { source.removeLast() }
        for line in source {
            if line.hasPrefix("diff --git ") {
                flush()
                header = [line]
            } else if line.hasPrefix("@@ ") {
                flush()
                lines = [line]
            } else if !lines.isEmpty {
                lines.append(line)
            } else {
                header.append(line)
            }
        }
        flush()
        return result
    }
}

public struct GitDiffRow: Identifiable, Sendable, Equatable {
    public let id: Int
    public let original: String
    public let modified: String
}

extension GitDiffParser {
    public static func pairedRows(_ hunk: GitDiffHunk) -> [GitDiffRow] {
        var result: [GitDiffRow] = []
        var removed: [String] = []
        var added: [String] = []
        func flush() {
            for index in 0..<max(removed.count, added.count) {
                result.append(
                    GitDiffRow(
                        id: result.count,
                        original: index < removed.count ? removed[index] : "",
                        modified: index < added.count ? added[index] : ""))
            }
            removed.removeAll()
            added.removeAll()
        }
        for line in hunk.lines.dropFirst() {
            if line.hasPrefix("-") {
                removed.append(line)
            } else if line.hasPrefix("+") {
                added.append(line)
            } else {
                flush()
                result.append(GitDiffRow(id: result.count, original: line, modified: line))
            }
        }
        flush()
        return result
    }
}