feat: lossless share links + auto iteration count

src/app.rs

@@ -124,6 +124,10 @@ pub struct FractalApp {
     power: u32,
     julia_c: (f64, f64),
     max_iterations: u32,
+    /// When set, `max_iterations` tracks the zoom depth automatically (so deep
+    /// zooms stay sharp without hand-tuning); the manual slider takes over when
+    /// unset. Turned off when a preset or share link supplies an explicit count.
+    auto_iterations: bool,
     color_scale: f32,
     color_offset: f32,
     palette: u32,
@@ -222,6 +226,7 @@ impl FractalApp {
             power: 3,
             julia_c: (-0.8, 0.156),
             max_iterations: 512,
+            auto_iterations: true,
             color_scale: 0.15,
             color_offset: 0.0,
             palette: 0,
@@ -338,16 +343,28 @@ impl FractalApp {
         {
             self.mode = FractalMode::Mandelbrot;
             self.view = ViewState::with_center(cre, cim, half_height);
+            // Presets carry a hand-tuned count; don't let the auto-scaler clobber it.
+            self.auto_iterations = false;
             self.max_iterations = iterations.clamp(32, MAX_REF_POINTS as u32 - 1);
         }
     }
 
+    /// Iteration count scaled to the current zoom depth, used while
+    /// `auto_iterations` is on. Grows roughly linearly with zoom decades so deep
+    /// zooms keep enough iterations to stay sharp instead of banding.
+    fn auto_iteration_count(&self) -> u32 {
+        let decades = self.view.magnification().log10().max(0.0);
+        let iters = 400.0 + 900.0 * decades;
+        (iters.round() as u32).clamp(200, MAX_REF_POINTS as u32 - 1)
+    }
+
     /// Snapshot the current view as a shareable state.
     fn share_state(&self) -> ShareState {
         let sig_digits = sig_digits_for(self.view.precision_bits());
         ShareState {
             julia: matches!(self.mode, FractalMode::Julia),
             kind: self.kind,
+            power: self.power,
             center_re: big_to_decimal_str(&self.view.center_re, sig_digits),
             center_im: big_to_decimal_str(&self.view.center_im, sig_digits),
             half_height: self.view.half_height,
@@ -355,6 +372,7 @@ impl FractalApp {
             julia_c: self.julia_c,
             color_scale: self.color_scale,
             color_offset: self.color_offset,
+            palette: self.palette,
         }
     }
 
@@ -366,9 +384,14 @@ impl FractalApp {
             FractalMode::Mandelbrot
         };
         self.kind = s.kind;
+        self.power = s.power.clamp(2, 8);
         self.julia_c = s.julia_c;
         self.color_scale = s.color_scale;
         self.color_offset = s.color_offset;
+        self.palette = (s.palette as usize).min(PALETTE_NAMES.len() - 1) as u32;
+        // The link carries an explicit iteration count; honor it rather than
+        // letting the auto-scaler immediately overwrite it.
+        self.auto_iterations = false;
         self.max_iterations = s.iterations.clamp(32, MAX_REF_POINTS as u32 - 1);
         let bits = precision_for(s.half_height);
         if let (Some(re), Some(im)) = (
@@ -808,11 +831,17 @@ impl FractalApp {
         }
 
         ui.separator();
+        ui.checkbox(&mut self.auto_iterations, "Auto iterations")
+            .on_hover_text("Scale the iteration count with zoom depth so deep zooms stay sharp.");
+        if self.auto_iterations {
+            ui.label(format!("iterations: {} (auto)", self.max_iterations));
+        } else {
             ui.add(
                 egui::Slider::new(&mut self.max_iterations, 32..=100_000)
                     .text("iterations")
                     .logarithmic(true),
             );
+        }
         ui.add(
             egui::Slider::new(&mut self.color_scale, 0.01..=1.0)
                 .text("color scale")
@@ -1006,6 +1035,11 @@ impl FractalApp {
             ui.ctx().request_repaint();
         }
 
+        // Keep the iteration count matched to the zoom depth while auto is on.
+        if self.auto_iterations {
+            self.max_iterations = self.auto_iteration_count();
+        }
+
         self.ensure_reference();
 
         // Poll the worker roughly every 30 ms while a reference is computing,

src/fractal/share.rs

@@ -14,6 +14,8 @@ use crate::fractal::FractalKind;
 pub struct ShareState {
     pub julia: bool,
     pub kind: FractalKind,
+    /// Exponent for the Multibrot kind (ignored by others).
+    pub power: u32,
     pub center_re: String,
     pub center_im: String,
     pub half_height: f64,
@@ -21,6 +23,8 @@ pub struct ShareState {
     pub julia_c: (f64, f64),
     pub color_scale: f32,
     pub color_offset: f32,
+    /// Palette index (`palette_id` in the shader).
+    pub palette: u32,
 }
 
 impl ShareState {
@@ -33,14 +37,15 @@ impl ShareState {
             FractalKind::Multibrot => "multi",
             FractalKind::Tricorn => "tricorn",
         }));
+        s.push_str(&format!("&pw={}", self.power));
         s.push_str(&format!(
             "&re={}&im={}&hh={}&it={}",
             self.center_re, self.center_im, self.half_height, self.iterations
         ));
         s.push_str(&format!("&jr={}&ji={}", self.julia_c.0, self.julia_c.1));
         s.push_str(&format!(
-            "&cs={}&co={}",
+            "&cs={}&co={}&pal={}",
-            self.color_scale, self.color_offset
+            self.color_scale, self.color_offset, self.palette
         ));
         s
     }
@@ -66,6 +71,7 @@ impl ShareState {
                     _ => FractalKind::Mandelbrot
                 })
                 .unwrap_or(FractalKind::Mandelbrot),
+            power: map.get("pw").and_then(|s| s.parse().ok()).unwrap_or(2),
             center_re: (*map.get("re")?).to_string(),
             center_im: (*map.get("im")?).to_string(),
             half_height: map.get("hh")?.parse().ok()?,
@@ -76,6 +82,7 @@ impl ShareState {
             ),
             color_scale: map.get("cs").and_then(|s| s.parse().ok()).unwrap_or(0.02),
             color_offset: map.get("co").and_then(|s| s.parse().ok()).unwrap_or(0.0),
+            palette: map.get("pal").and_then(|s| s.parse().ok()).unwrap_or(0),
         })
     }
 }
@@ -88,7 +95,8 @@ mod tests {
     fn round_trip() {
         let s = ShareState {
             julia: true,
-            kind: FractalKind::Tricorn,
+            kind: FractalKind::Multibrot,
+            power: 5,
             center_re: "-0.743643887037158704752191506114774".into(),
             center_im: "0.131825904205311970493132056385139".into(),
             half_height: 1.5e-20,
@@ -96,15 +104,18 @@ mod tests {
             julia_c: (-0.123, 0.745),
             color_scale: 0.02,
             color_offset: 0.25,
+            palette: 3,
         };
         let d = ShareState::decode(&s.encode()).unwrap();
         assert_eq!(d.julia, s.julia);
         assert_eq!(d.kind, s.kind);
+        assert_eq!(d.power, s.power);
         assert_eq!(d.center_re, s.center_re);
         assert_eq!(d.center_im, s.center_im);
         assert_eq!(d.half_height, s.half_height);
         assert_eq!(d.iterations, s.iterations);
         assert_eq!(d.julia_c, s.julia_c);
+        assert_eq!(d.palette, s.palette);
     }
 
     #[test]