Expressions: read bezier curve by position (#1884)

A missing or unusable curve no longer consumes the last value. The long form reads argument 5 and the short form argument 3, with the other arities falling back to linear interpolation. Adds the missing-curve and four-element-array cases reported in review.

crates/expr/src/prelude.js

@@ -251,13 +251,14 @@ function __cubicBezier(curve) {
 }
 
 // bezier(t, tMin, tMax, value1, value2, curve), or bezier(t, value1, value2, curve): the curve is
-// the last argument; everything before it follows linear / ease.
+// the sixth / fourth argument. Read it by position, not from the end: when it is missing or
+// unusable, the interpolation falls back to linear without consuming the last value.
 function bezier() {
-  var curve = arguments[arguments.length - 1];
-  var shape = __cubicBezier(curve);
-  var args = [];
-  for (var i = 0; i < arguments.length - 1; i++) args.push(arguments[i]);
-  return __interp(shape, args);
+  var a = arguments, n = a.length;
+  if (n >= 6) return __interp(__cubicBezier(a[5]), [a[0], a[1], a[2], a[3], a[4]]);
+  if (n === 5) return __interp(__lin, [a[0], a[1], a[2], a[3], a[4]]);
+  if (n === 4) return __interp(__cubicBezier(a[3]), [a[0], a[1], a[2]]);
+  return __interp(__lin, [a[0], a[1], a[2]]);
 }
 
 // ---------------------------------------------------------------------------------------------

crates/expr/src/tests.rs

@@ -343,6 +343,12 @@ fn cubic_bezier_short_form_arrays_and_fallbacks() {
     // A curve that is not four finite numbers falls back to linear instead of throwing.
     assert_close!(f.num("bezier(0.5, 0, 1, 0, 100, [1, 0])", 0.0), 50.0);
     assert_close!(f.num("bezier(0.5, 0, 1, 0, 100, 'ease')", 0.0), 50.0);
+    // A missing or unusable curve must not consume the last value: read the curve by position.
+    assert_close!(f.num("bezier(0.5, 0, 1, 0, 100)", 0.0), 50.0);
+    assert_close!(f.num("bezier(0.5, 0, 100)", 0.0), 50.0);
+    assert_close!(f.num("bezier(0.5, 0, 100, [1, 0])", 0.0), 50.0);
+    let v = f.ev(f.shape, "contents/#1/contents/fill/color", "bezier(0.5, 0, 1, [0, 0, 0, 1], [1, 1, 1, 1])", 0.0).unwrap();
+    assert_eq!(v, Value::Color([0.5, 0.5, 0.5, 1.0]));
     // Exactly four values are a curve; extra ones mean the curve is malformed, so linear.
     assert_close!(f.num("bezier(0.5, 0, 1, 0, 100, [0.42, 0, 0.58, 1, 0])", 0.0), 50.0);
     assert_close!(f.num("bezier(0.5, 0, 1, 0, 100, [0.42, 0, 0.58, 1, 2, 3])", 0.0), 50.0);