feat(svg): SVG path-data parser for the drawing-card renderer (T-320)
Parse the `d` attribute into an absolute-coordinate SvgPathSeg list, lowering SVG shorthands to a small command set the painter can replay: relative→absolute, H/V→lineTo, S→cubic and T→quad with control reflection, arcs kept as arcTo. Tolerant — malformed tail returns the understood prefix and never throws, so a bad diagram can't crash the conversation. Flutter-free, covered by dart test. First slice of the SVG engine (D-103); no user-visible behaviour yet. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
Claude Opus 4.8
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/// SVG `path` `d`-attribute parser (T-320 / D-103).
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///
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/// Parses SVG path data into a flat list of [SvgPathSeg]s in ABSOLUTE
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/// coordinates, lowering the SVG shorthands to a small command set the painter
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/// can replay straight onto a `dart:ui` Path:
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/// - relative (lower-case) commands → absolute
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/// - `H`/`V` → [SvgPathOp.lineTo]
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/// - `S` → [SvgPathOp.cubicTo] (first control reflected from the previous cubic)
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/// - `T` → [SvgPathOp.quadTo] (control reflected from the previous quadratic)
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/// - arcs (`A`) kept as [SvgPathOp.arcTo] for the painter to realise
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///
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/// Tolerant by construction: malformed tail input stops the parse and returns
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/// what was understood so far — a broken `d` never throws, so a bad diagram
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/// can't crash the conversation. No dependency, per prefer-zero-deps.
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///
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/// Flutter-free: pure Dart, runs under `dart test`. The painter (Flutter)
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/// consumes [SvgPathSeg] and builds the actual Path.
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library;
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/// The painter-facing path command set the parser lowers SVG path data to.
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enum SvgPathOp { moveTo, lineTo, cubicTo, quadTo, arcTo, close }
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/// One absolute-coordinate path segment.
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///
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/// [args] layout by [op]:
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/// - moveTo / lineTo : `[x, y]`
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/// - cubicTo : `[x1, y1, x2, y2, x, y]`
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/// - quadTo : `[x1, y1, x, y]`
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/// - arcTo : `[rx, ry, xAxisRotationDeg, largeArc, sweep, x, y]`
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/// (largeArc / sweep are `0.0` or `1.0`)
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/// - close : `[]`
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class SvgPathSeg {
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final SvgPathOp op;
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final List<double> args;
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const SvgPathSeg(this.op, this.args);
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@override
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bool operator ==(Object other) {
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if (other is! SvgPathSeg || other.op != op || other.args.length != args.length) {
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return false;
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}
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for (var i = 0; i < args.length; i++) {
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if (other.args[i] != args[i]) return false;
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}
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return true;
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}
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@override
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int get hashCode => Object.hash(op, Object.hashAll(args));
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@override
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String toString() => '${op.name}(${args.join(', ')})';
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}
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/// Parse an SVG `path` `d` attribute into absolute segments. Returns whatever
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/// was understood up to the first unrecoverable token; never throws.
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List<SvgPathSeg> parseSvgPath(String d) {
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final out = <SvgPathSeg>[];
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final sc = _Scanner(d);
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double cx = 0, cy = 0; // current point
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double sx = 0, sy = 0; // subpath start (target of Z)
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double? cubX, cubY; // previous cubic's 2nd control (for S reflection)
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double? quadX, quadY; // previous quadratic's control (for T reflection)
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String prev = '';
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sc.skipSep();
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while (!sc.atEnd) {
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final loopStart = sc.i;
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final ch = sc.s[sc.i];
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final explicit = _isCmd(ch);
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String cmd;
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if (explicit) {
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cmd = ch;
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sc.i++;
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} else if (prev.isNotEmpty) {
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// Implicit repeat of the previous command; an initial M/m repeats as L/l.
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cmd = prev == 'M' ? 'L' : (prev == 'm' ? 'l' : prev);
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} else {
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break; // data must open with a command
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}
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// Z carries no parameters, so it never repeats implicitly.
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if (!explicit && (cmd == 'Z' || cmd == 'z')) break;
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final rel = cmd == cmd.toLowerCase();
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switch (cmd.toUpperCase()) {
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case 'M':
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final x = sc.num_(), y = sc.num_();
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if (x == null || y == null) return out;
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cx = rel ? cx + x : x;
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cy = rel ? cy + y : y;
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sx = cx;
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sy = cy;
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out.add(SvgPathSeg(SvgPathOp.moveTo, [cx, cy]));
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cubX = cubY = quadX = quadY = null;
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case 'L':
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final x = sc.num_(), y = sc.num_();
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if (x == null || y == null) return out;
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cx = rel ? cx + x : x;
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cy = rel ? cy + y : y;
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out.add(SvgPathSeg(SvgPathOp.lineTo, [cx, cy]));
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cubX = cubY = quadX = quadY = null;
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case 'H':
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final x = sc.num_();
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if (x == null) return out;
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cx = rel ? cx + x : x;
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out.add(SvgPathSeg(SvgPathOp.lineTo, [cx, cy]));
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cubX = cubY = quadX = quadY = null;
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case 'V':
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final y = sc.num_();
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if (y == null) return out;
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cy = rel ? cy + y : y;
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out.add(SvgPathSeg(SvgPathOp.lineTo, [cx, cy]));
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cubX = cubY = quadX = quadY = null;
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case 'C':
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final x1 = sc.num_(), y1 = sc.num_();
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final x2 = sc.num_(), y2 = sc.num_();
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final x = sc.num_(), y = sc.num_();
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if (x1 == null || y1 == null || x2 == null || y2 == null || x == null || y == null) {
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return out;
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}
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final ax1 = rel ? cx + x1 : x1, ay1 = rel ? cy + y1 : y1;
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final ax2 = rel ? cx + x2 : x2, ay2 = rel ? cy + y2 : y2;
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final ax = rel ? cx + x : x, ay = rel ? cy + y : y;
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out.add(SvgPathSeg(SvgPathOp.cubicTo, [ax1, ay1, ax2, ay2, ax, ay]));
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cubX = ax2;
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cubY = ay2;
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quadX = quadY = null;
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cx = ax;
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cy = ay;
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case 'S':
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final x2 = sc.num_(), y2 = sc.num_();
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final x = sc.num_(), y = sc.num_();
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if (x2 == null || y2 == null || x == null || y == null) return out;
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final rx = cubX != null ? 2 * cx - cubX : cx;
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final ry = cubY != null ? 2 * cy - cubY : cy;
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final ax2 = rel ? cx + x2 : x2, ay2 = rel ? cy + y2 : y2;
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final ax = rel ? cx + x : x, ay = rel ? cy + y : y;
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out.add(SvgPathSeg(SvgPathOp.cubicTo, [rx, ry, ax2, ay2, ax, ay]));
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cubX = ax2;
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cubY = ay2;
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quadX = quadY = null;
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cx = ax;
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cy = ay;
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case 'Q':
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final x1 = sc.num_(), y1 = sc.num_();
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final x = sc.num_(), y = sc.num_();
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if (x1 == null || y1 == null || x == null || y == null) return out;
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final ax1 = rel ? cx + x1 : x1, ay1 = rel ? cy + y1 : y1;
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final ax = rel ? cx + x : x, ay = rel ? cy + y : y;
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out.add(SvgPathSeg(SvgPathOp.quadTo, [ax1, ay1, ax, ay]));
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quadX = ax1;
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quadY = ay1;
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cubX = cubY = null;
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cx = ax;
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cy = ay;
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case 'T':
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final x = sc.num_(), y = sc.num_();
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if (x == null || y == null) return out;
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final rx = quadX != null ? 2 * cx - quadX : cx;
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final ry = quadY != null ? 2 * cy - quadY : cy;
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final ax = rel ? cx + x : x, ay = rel ? cy + y : y;
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out.add(SvgPathSeg(SvgPathOp.quadTo, [rx, ry, ax, ay]));
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quadX = rx;
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quadY = ry;
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cubX = cubY = null;
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cx = ax;
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cy = ay;
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case 'A':
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final rx = sc.num_(), ry = sc.num_(), rot = sc.num_();
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final laf = sc.flag(), swf = sc.flag();
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final x = sc.num_(), y = sc.num_();
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if (rx == null || ry == null || rot == null || laf == null || swf == null || x == null || y == null) {
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return out;
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}
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final ax = rel ? cx + x : x, ay = rel ? cy + y : y;
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out.add(SvgPathSeg(SvgPathOp.arcTo, [rx.abs(), ry.abs(), rot, laf.toDouble(), swf.toDouble(), ax, ay]));
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cubX = cubY = quadX = quadY = null;
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cx = ax;
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cy = ay;
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case 'Z':
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out.add(const SvgPathSeg(SvgPathOp.close, []));
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cx = sx;
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cy = sy;
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cubX = cubY = quadX = quadY = null;
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default:
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return out; // unknown command letter
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}
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prev = cmd;
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sc.skipSep();
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if (sc.i == loopStart) break; // no progress (e.g. implicit repeat of Z) — bail
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}
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return out;
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}
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bool _isCmd(String c) => 'MmLlHhVvCcSsQqTtAaZz'.contains(c);
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bool _isDigit(int c) => c >= 0x30 && c <= 0x39;
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/// A forgiving cursor over path data, handling SVG's wsp/comma separators and
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/// its delimiter-free number packing (`.5.5`, `1-2`, `1e2`).
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class _Scanner {
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_Scanner(this.s);
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final String s;
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int i = 0;
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bool get atEnd => i >= s.length;
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void skipSep() {
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while (i < s.length) {
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final c = s.codeUnitAt(i);
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// wsp (space, tab, LF, CR, FF) or comma
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if (c == 0x20 || c == 0x09 || c == 0x0A || c == 0x0D || c == 0x0C || c == 0x2C) {
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i++;
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} else {
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break;
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}
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}
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}
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/// Read a number; `null` (cursor restored) if none is present.
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double? num_() {
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skipSep();
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final start = i;
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if (i < s.length && (s[i] == '+' || s[i] == '-')) i++;
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var digits = false;
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while (i < s.length && _isDigit(s.codeUnitAt(i))) {
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i++;
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digits = true;
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}
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if (i < s.length && s[i] == '.') {
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i++;
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while (i < s.length && _isDigit(s.codeUnitAt(i))) {
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i++;
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digits = true;
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}
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}
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if (!digits) {
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i = start;
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return null;
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}
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if (i < s.length && (s[i] == 'e' || s[i] == 'E')) {
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final save = i;
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i++;
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if (i < s.length && (s[i] == '+' || s[i] == '-')) i++;
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var expDigits = false;
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while (i < s.length && _isDigit(s.codeUnitAt(i))) {
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i++;
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expDigits = true;
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}
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if (!expDigits) i = save; // bare 'e' — not part of the number
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}
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return double.tryParse(s.substring(start, i));
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}
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/// Arc flags are a single `0` or `1`, possibly packed against neighbours.
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int? flag() {
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skipSep();
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if (i < s.length && (s[i] == '0' || s[i] == '1')) {
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final v = s[i] == '1' ? 1 : 0;
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i++;
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return v;
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}
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return null;
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}
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}
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