fix: preview D4 paylow/paymed .tex from the payload twin

D4 paylow/ and paymed/ files are mip-stream variants whose byte layout
isn't reverse-engineered yet, so the brute-force BC decoder can't recover
power-of-2 dimensions and the preview pane just shows "could not be
decoded". When that happens, fetch the matching base/payload/<rest> file
via SiblingFetcher and preview that instead, with a note in the text
panel calling out what the user is actually looking at. Export Raw is
unaffected (framework-level) so the original paylow bytes are still what
gets written. PNG export uses the twin bytes too, since it should mirror
what's on screen.

Author: jabberwock

rustydemon/src/preview/tex.rs

@@ -20,37 +20,133 @@ impl PreviewPlugin for TexPreview {
         filename: &str,
         data: &[u8],
         ctx: &egui::Context,
-        _fetch: &super::SiblingFetcher<'_>,
+        fetch: &super::SiblingFetcher<'_>,
     ) -> PreviewOutput {
-        let mut out = PreviewOutput::new();
+        // Try the file as-is first. paylow/paymed are mip-stream variants we
+        // can't decode yet, so on failure we fall back to previewing the
+        // matching payload/ twin. Export Raw still writes the original bytes.
+        if let Some((rgba, w, h, fmt)) = crate::tex_preview::decode_tex(data, filename) {
+            return build_output(ctx, rgba, w, h, fmt, filename.to_owned(), None, None);
+        }
+
+        if let Some((kind, twin_path)) = payload_twin(filename) {
+            if let Some(twin_bytes) = (fetch.by_name)(&twin_path) {
+                if let Some((rgba, w, h, fmt)) =
+                    crate::tex_preview::decode_tex(&twin_bytes, &twin_path)
+                {
+                    let note = format!(
+                        "{kind}/ mipmap stream isn't decoded yet — preview is from \
+                         the matching payload/ variant. Export Raw still writes \
+                         the original {kind} bytes."
+                    );
+                    return build_output(
+                        ctx,
+                        rgba,
+                        w,
+                        h,
+                        fmt,
+                        twin_path,
+                        Some(twin_bytes),
+                        Some(note),
+                    );
+                }
+            }
+        }
 
-        let Some((rgba, w, h, fmt)) = crate::tex_preview::decode_tex(data, filename) else {
-            out.text = Some(
-                ".tex header could not be decoded — unknown dimensions or unsupported BC format."
-                    .into(),
-            );
-            return out;
-        };
-
-        let color_image = egui::ColorImage::from_rgba_unmultiplied([w as usize, h as usize], &rgba);
-        out.texture =
-            Some(ctx.load_texture("tex_preview", color_image, egui::TextureOptions::default()));
-        out.texture_pixels = Some((rgba, w, h));
-        out.text = Some(format!(
-            "D4 .tex texture\n{w}×{h}  {fmt}\n\nDecoded from raw block-compressed data."
-        ));
-
-        let filename = filename.to_owned();
-        out.extra_exports.push(ExportAction {
-            label: "Export As PNG",
-            default_extension: "png",
-            filter_name: "PNG image",
-            build: Arc::new(move |data, _path| {
-                let (rgba, w, h, _fmt) = crate::tex_preview::decode_tex(data, &filename)
-                    .ok_or_else(|| "tex decode failed".to_string())?;
-                crate::preview::encode_png(&rgba, w, h)
-            }),
-        });
+        let mut out = PreviewOutput::new();
+        out.text = Some(
+            ".tex header could not be decoded — unknown dimensions or unsupported BC format."
+                .into(),
+        );
         out
     }
 }
+
+/// If `filename` lives under `base/paylow/` or `base/paymed/`, return
+/// `(tier_label, twin_path_under_base/payload/)` so the plugin can fetch
+/// the full-resolution variant for preview.
+fn payload_twin(filename: &str) -> Option<(&'static str, String)> {
+    let lower = filename.to_ascii_lowercase();
+    if let Some(rest) = lower.strip_prefix("base/paylow/") {
+        Some(("paylow", format!("base/payload/{rest}")))
+    } else if let Some(rest) = lower.strip_prefix("base/paymed/") {
+        Some(("paymed", format!("base/payload/{rest}")))
+    } else {
+        None
+    }
+}
+
+#[allow(clippy::too_many_arguments)]
+fn build_output(
+    ctx: &egui::Context,
+    rgba: Vec<u8>,
+    w: u32,
+    h: u32,
+    fmt: &'static str,
+    decode_filename: String,
+    decoded_bytes_for_export: Option<Vec<u8>>,
+    note: Option<String>,
+) -> PreviewOutput {
+    let mut out = PreviewOutput::new();
+    let color_image = egui::ColorImage::from_rgba_unmultiplied([w as usize, h as usize], &rgba);
+    out.texture =
+        Some(ctx.load_texture("tex_preview", color_image, egui::TextureOptions::default()));
+    out.texture_pixels = Some((rgba, w, h));
+
+    let mut text =
+        format!("D4 .tex texture\n{w}×{h}  {fmt}\n\nDecoded from raw block-compressed data.");
+    if let Some(n) = note {
+        text.push_str("\n\n");
+        text.push_str(&n);
+    }
+    out.text = Some(text);
+
+    out.extra_exports.push(ExportAction {
+        label: "Export As PNG",
+        default_extension: "png",
+        filter_name: "PNG image",
+        build: Arc::new(move |data, _path| {
+            // PNG export mirrors what the user *sees*. When we previewed a
+            // payload twin, encode that; otherwise fall back to the file
+            // bytes the framework hands us.
+            let bytes: &[u8] = decoded_bytes_for_export.as_deref().unwrap_or(data);
+            let (rgba, w, h, _fmt) = crate::tex_preview::decode_tex(bytes, &decode_filename)
+                .ok_or_else(|| "tex decode failed".to_string())?;
+            crate::preview::encode_png(&rgba, w, h)
+        }),
+    });
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::payload_twin;
+
+    #[test]
+    fn paylow_maps_to_payload_twin() {
+        let (tier, twin) =
+            payload_twin("base/paylow/Texture/warlock_sigilOfSummons_Color.tex").unwrap();
+        assert_eq!(tier, "paylow");
+        assert_eq!(
+            twin,
+            "base/payload/texture/warlock_sigilofsummons_color.tex"
+        );
+    }
+
+    #[test]
+    fn paymed_maps_to_payload_twin() {
+        let (tier, twin) = payload_twin("base/paymed/Texture/some_file.tex").unwrap();
+        assert_eq!(tier, "paymed");
+        assert_eq!(twin, "base/payload/texture/some_file.tex");
+    }
+
+    #[test]
+    fn payload_returns_none() {
+        assert!(payload_twin("base/payload/Texture/foo.tex").is_none());
+    }
+
+    #[test]
+    fn unrelated_path_returns_none() {
+        assert!(payload_twin("World/Maps/Azeroth/foo.adt").is_none());
+    }
+}