import "package:diff_match_patch/diff_match_patch.dart";
import "package:shadcn_flutter/shadcn_flutter.dart";

const _contextLines = 3;

class DiffView extends StatelessWidget {
  const DiffView({
    super.key,
    this.oldString,
    this.newString,
    this.content,
  }) : assert(
          content != null || (oldString != null && newString != null),
          "Provide either content (view-only) or oldString+newString (diff)",
        );

  final String? oldString;
  final String? newString;

  // view-only mode — show content as plain code, no diff colors
  final String? content;

  @override
  Widget build(BuildContext context) {
    if (content != null) {
      final lines = content!.split("\n");
      final viewLines = [
        for (int i = 0; i < lines.length; i++)
          _DiffLine(_LineKind.context, lines[i], newLine: i + 1),
      ];
      final hunk = _Hunk(oldStart: 1, newStart: 1, lines: viewLines);
      return _HunkView(hunk: hunk);
    }

    final hunks = _computeHunks(oldString!, newString!);

    if (hunks.isEmpty) return const SizedBox.shrink();

    return Column(
      crossAxisAlignment: CrossAxisAlignment.stretch,
      children: [
        for (final hunk in hunks) ...[

          // is first
          if (hunk != hunks.first) ...[
            Divider(),
            Gap(1),
            Divider(),
          ],

          _HunkView(hunk: hunk)
        ],
      ],
    );
  }
}

// ─── data model ───────────────────────────────────────────────────────────────

enum _LineKind { context, added, removed }

class _DiffLine {
  const _DiffLine(this.kind, this.text, {this.oldLine, this.newLine});
  final _LineKind kind;
  final String text;
  final int? oldLine;
  final int? newLine;
}

class _Hunk {
  _Hunk({
    required this.oldStart,
    required this.newStart,
    required this.lines,
  });
  final int oldStart;
  final int newStart;
  final List<_DiffLine> lines;

  int get oldCount => lines.where((l) => l.kind != _LineKind.added).length;
  int get newCount => lines.where((l) => l.kind != _LineKind.removed).length;
}

// ─── diff computation ─────────────────────────────────────────────────────────

List<_Hunk> _computeHunks(String oldStr, String newStr) {
  final dmp = DiffMatchPatch();

  final oldLines = oldStr.split("\n");
  final newLines = newStr.split("\n");

  // encode lines → single chars so dmp does line-level diff
  final enc = _encodeLines(oldLines, newLines);
  final diffs = dmp.diff(enc.oldEncoded, enc.newEncoded, false);
  dmp.diffCleanupSemantic(diffs);

  // expand diffs back to line sequences
  final rawLines = <_DiffLine>[];
  int oldIdx = 0;
  int newIdx = 0;

  for (final d in diffs) {
    final count = d.text.length; // each char == one line
    switch (d.operation) {
      case DIFF_EQUAL:
        for (int i = 0; i < count; i++) {
          rawLines.add(_DiffLine(
            _LineKind.context,
            enc.lines[d.text.codeUnitAt(i) - 0xE000],
            oldLine: oldIdx + 1,
            newLine: newIdx + 1,
          ));
          oldIdx++;
          newIdx++;
        }
        break;

      case DIFF_DELETE:
        for (int i = 0; i < count; i++) {
          rawLines.add(_DiffLine(
            _LineKind.removed,
            enc.lines[d.text.codeUnitAt(i) - 0xE000],
            oldLine: oldIdx + 1,
          ));
          oldIdx++;
        }
        break;

      case DIFF_INSERT:
        for (int i = 0; i < count; i++) {
          rawLines.add(_DiffLine(
            _LineKind.added,
            enc.lines[d.text.codeUnitAt(i) - 0xE000],
            newLine: newIdx + 1,
          ));
          newIdx++;
        }
        break;
    }
  }

  return _groupIntoHunks(rawLines);
}

// keep only context lines that are within _contextLines of a change
List<_Hunk> _groupIntoHunks(List<_DiffLine> rawLines) {
  final n = rawLines.length;

  // mark which context lines to keep
  final keep = List<bool>.filled(n, false);
  for (int i = 0; i < n; i++) {
    if (rawLines[i].kind != _LineKind.context) {
      for (int j = (i - _contextLines).clamp(0, n - 1);
          j <= (i + _contextLines).clamp(0, n - 1);
          j++) {
        keep[j] = true;
      }
    }
  }

  final hunks = <_Hunk>[];
  int i = 0;

  while (i < n) {
    if (!keep[i]) {
      i++;
      continue;
    }

    // start of a new hunk
    final hunkLines = <_DiffLine>[];
    int oldStart = rawLines[i].oldLine ?? 1;
    int newStart = rawLines[i].newLine ?? 1;

    while (i < n && keep[i]) {
      hunkLines.add(rawLines[i]);
      i++;
    }

    hunks.add(_Hunk(
      oldStart: oldStart,
      newStart: newStart,
      lines: hunkLines,
    ));
  }

  return hunks;
}

// line encoding — maps unique lines to single unicode chars starting at U+E000
class _LineEncoding {
  final List<String> lines; // index → line text
  final String oldEncoded;
  final String newEncoded;
  const _LineEncoding(this.lines, this.oldEncoded, this.newEncoded);
}

_LineEncoding _encodeLines(List<String> oldLines, List<String> newLines) {
  final lineIndex = <String, int>{};
  final lines = <String>[];

  String encode(List<String> src) {
    final buf = StringBuffer();
    for (final line in src) {
      if (!lineIndex.containsKey(line)) {
        lineIndex[line] = lines.length;
        lines.add(line);
      }
      buf.writeCharCode(0xE000 + lineIndex[line]!);
    }
    return buf.toString();
  }

  final oldEncoded = encode(oldLines);
  final newEncoded = encode(newLines);
  return _LineEncoding(lines, oldEncoded, newEncoded);
}

// ─── widgets ──────────────────────────────────────────────────────────────────

String _hunkSummary(_Hunk hunk) {
  final added = hunk.lines.where((l) => l.kind == _LineKind.added).length;
  final removed = hunk.lines.where((l) => l.kind == _LineKind.removed).length;

  final parts = <String>[];
  if (added > 0) parts.add("Added $added ${added == 1 ? 'line' : 'lines'}");
  if (removed > 0) parts.add("removed $removed ${removed == 1 ? 'line' : 'lines'}");

  return parts.join(", ");
}

class _HunkView extends StatelessWidget {
  const _HunkView({required this.hunk});
  final _Hunk hunk;

  @override
  Widget build(BuildContext context) {
    final theme = Theme.of(context);

    return Column(
      crossAxisAlignment: CrossAxisAlignment.stretch,
      children: [

        // hunk header
        Container(
          // color: theme.colorScheme.muted.withValues(alpha: 0.4),
          padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 6),
          child: Text(
            _hunkSummary(hunk),
            style: TextStyle(color: theme.colorScheme.mutedForeground),
          ).xSmall.mono,
        ),

        Divider(),

        for (final line in hunk.lines) _LineView(line: line),

      ],
    );
  }
}

class _LineView extends StatelessWidget {
  const _LineView({required this.line});
  final _DiffLine line;

  @override
  Widget build(BuildContext context) {
    final theme = Theme.of(context);

    final Color bg;
    final Color fg;
    final String prefix;

    switch (line.kind) {
      case _LineKind.added:
        bg = const Color(0xFF166534).withValues(alpha: 0.2);
        fg = const Color(0xFF4ADE80);
        prefix = "+";
        break;
      case _LineKind.removed:
        bg = const Color(0xFF991B1B).withValues(alpha: 0.2);
        fg = const Color(0xFFF87171);
        prefix = "-";
        break;
      case _LineKind.context:
        bg = Colors.transparent;
        fg = theme.colorScheme.mutedForeground;
        prefix = " ";
        break;
    }

    final numColor = theme.colorScheme.mutedForeground.withValues(alpha: 0.5);
    final lineNum = line.kind == _LineKind.removed ? line.oldLine : line.newLine;

    return Container(
      color: bg,
      padding: const EdgeInsets.symmetric(vertical: 1),
      child: Row(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [

          SizedBox(
            width: 32,
            child: Text(
              lineNum != null ? "$lineNum" : "",
              textAlign: TextAlign.right,
              style: TextStyle(color: numColor),
            ).mono.xSmall,
          ),

          const SizedBox(width: 8),

          SizedBox(
            width: 10,
            child: Text(prefix, style: TextStyle(color: fg)).mono.xSmall,
          ),

          const SizedBox(width: 4),

          Expanded(
            child: Text(line.text, style: TextStyle(color: fg)).mono.xSmall,
          ),

        ],
      ),
    );
  }
}