Legacy .ai export: a radial gradient's color stops are written from ramp point 0 to 100

The version 7 .ai writer wrote the %_Bs color stops of every gradient
from ramp point 100 down to 0. The Adobe Illustrator File Format
Specification version 7.0 lists a linear gradient's stops in that order
(section 1.4.3). Its radial definition, "White & Purple Radial", lists
them from 10 to 100 (section 3.5.2). As reported on #2807, the
reference app reads both stops of #2687's radial gradient, written from
100 down to 0, at ramp point 100 and draws the circle in #1b2a6b. It
reads a radial gradient with three stops written that way in reverse
order.

The writer now lists a radial gradient's stops from the first to the
last. Each stop is written with its own midpoint to the next stop, and
section 3.5.2 says the last stop's midpoint is ignored. Linear
gradients are unchanged.

crates/engine/src/tests_legacy_ai.rs

@@ -204,8 +204,8 @@ const RADIAL_AND_LINEAR: &str = r##"<svg xmlns="http://www.w3.org/2000/svg" widt
 </svg>"##;
 
 /// A radial gradient is written with its highlight, `0 0 0 0 Bh` at the center (§3.5.5, §3.5.6).
-/// #2687's file had none, and the reference app drew the circle in its outer color. The linear one
-/// has none. Both open again with their type, stops and place.
+/// #2687's file had none. The linear one has none. Both open again with their type, stops and
+/// place.
 #[test]
 fn a_radial_gradient_is_written_with_its_highlight() {
     let mut s = Session::new();
@@ -244,6 +244,34 @@ fn a_radial_gradient_is_written_with_its_highlight() {
     }
 }
 
+/// #2687's radial gradient lists its stops from white at ramp point 0 to #1b2a6b at 100, as
+/// §3.5.2's radial definition lists them (10, then 100); the linear one lists them from 100 to 0
+/// (§1.4.3). The reference app read the radial gradient's two stops listed from 100 both at ramp
+/// point 100 and drew the circle in #1b2a6b (reported on #2807).
+#[test]
+fn a_radial_gradient_lists_its_stops_from_the_first() {
+    let mut s = Session::new();
+    let svg = vectorcraft_format::base64_encode(RADIAL_AND_LINEAR.as_bytes());
+    s.execute("document.open", &json!({"name": "t_radial.svg", "dataBase64": svg})).unwrap();
+    let r = s.execute("file.saveCopy", &json!({"format": "ai", "aiVersion": "7"})).unwrap();
+    let text = String::from_utf8(vectorcraft_format::base64_decode(r["dataBase64"].as_str().unwrap()).unwrap()).unwrap();
+    // Each `%_Bs` line of a definition as its RGB color (0-255) and ramp point.
+    let listed = |head: &str| -> Vec<([u8; 3], String)> {
+        let def = text.split(head).nth(1).and_then(|t| t.split("BD\n").next()).unwrap_or_else(|| panic!("no {head:?} in\n{text}"));
+        def.lines()
+            .map(|l| {
+                let n: Vec<&str> = l.split_whitespace().collect();
+                let [_, _, _, _, r, g, b, "2", "50", ramp, "%_Bs"] = n.as_slice() else { panic!("{l}") };
+                let c = [r, g, b].map(|v| (v.parse::<f64>().unwrap() * 255.0).round() as u8);
+                (c, (*ramp).to_string())
+            })
+            .collect()
+    };
+    let (white, navy) = ([255, 255, 255], [27, 42, 107]);
+    assert_eq!(listed("(Gradient) 1 2 Bd\n[\n"), [(white, "0".to_string()), (navy, "100".to_string())], "{text}");
+    assert_eq!(listed("(Gradient 2) 0 2 Bd\n[\n"), [(navy, "100".to_string()), (white, "0".to_string())], "{text}");
+}
+
 /// A `w`×`h` PNG of one colour (`rgba`), 72 ppi.
 fn png(w: u32, h: u32, rgba: [u8; 4]) -> Vec<u8> {
     let mut out = vec![];