From 9927046c7e45683a5d6f7e8f55c522560057236c Mon Sep 17 00:00:00 2001
From: Vassiliy Yegorov <va@egorovanet.ru>
Date: Tue, 9 Jun 2026 21:15:47 +0700
Subject: [PATCH] feat(core): 10 layout preset generators

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 crates/spacesh-core/src/lib.rs     |   1 +
 crates/spacesh-core/src/presets.rs | 102 +++++++++++++++++++++++++++++
 2 files changed, 103 insertions(+)
 create mode 100644 crates/spacesh-core/src/presets.rs

diff --git a/crates/spacesh-core/src/lib.rs b/crates/spacesh-core/src/lib.rs
index ff47eea..04b0164 100644
--- a/crates/spacesh-core/src/lib.rs
+++ b/crates/spacesh-core/src/lib.rs
@@ -1,5 +1,6 @@
 pub mod grid;
 pub mod ops;
+pub mod presets;
 pub mod snapshot;
 
 pub use grid::GridSurface;
diff --git a/crates/spacesh-core/src/presets.rs b/crates/spacesh-core/src/presets.rs
new file mode 100644
index 0000000..6928dcd
--- /dev/null
+++ b/crates/spacesh-core/src/presets.rs
@@ -0,0 +1,102 @@
+//! The 10 layout presets (DOCS/MAIN.md §8.2). A preset maps a list of surface
+//! ids (one per slot, in order) to a LayoutNode. `slot_count` says how many
+//! panels the preset needs.
+use spacesh_proto::ids::SurfaceId;
+use spacesh_proto::layout::{LayoutNode, Orient};
+
+/// Known preset ids and their panel counts.
+pub fn slot_count(preset_id: &str) -> Option<usize> {
+    Some(match preset_id {
+        "1" => 1,
+        "2lr" => 2,   // 2↔
+        "2tb" => 2,   // 2↕
+        "2+1" => 3,
+        "1+2" => 3,
+        "3" => 3,
+        "2x2" => 4,
+        "4" => 4,     // single row of 4
+        "2x3" => 6,
+        "2x4" => 8,
+        _ => return None,
+    })
+}
+
+fn leaf(id: &SurfaceId) -> LayoutNode { LayoutNode::leaf(id.clone()) }
+fn even(n: usize) -> Vec<f32> { vec![1.0 / n as f32; n] }
+
+fn row(ids: &[SurfaceId]) -> LayoutNode {
+    LayoutNode::Split { orient: Orient::H, ratios: even(ids.len()), children: ids.iter().map(leaf).collect() }
+}
+fn col(children: Vec<LayoutNode>) -> LayoutNode {
+    LayoutNode::Split { orient: Orient::V, ratios: even(children.len()), children }
+}
+fn rown(children: Vec<LayoutNode>) -> LayoutNode {
+    LayoutNode::Split { orient: Orient::H, ratios: even(children.len()), children }
+}
+
+/// Build the preset tree from exactly `slot_count(preset_id)` ids.
+/// Returns None for an unknown id or wrong id count.
+pub fn build(preset_id: &str, ids: &[SurfaceId]) -> Option<LayoutNode> {
+    if slot_count(preset_id)? != ids.len() {
+        return None;
+    }
+    Some(match preset_id {
+        "1" => leaf(&ids[0]),
+        "2lr" => row(&ids),
+        "2tb" => col(vec![leaf(&ids[0]), leaf(&ids[1])]),
+        // left big column over... 2 stacked on the right.
+        "2+1" => rown(vec![leaf(&ids[0]), col(vec![leaf(&ids[1]), leaf(&ids[2])])]),
+        // one big on the left, 2 stacked on the right (mirror naming kept simple).
+        "1+2" => rown(vec![col(vec![leaf(&ids[0]), leaf(&ids[1])]), leaf(&ids[2])]),
+        "3" => row(&ids),
+        "2x2" => col(vec![row(&ids[0..2]), row(&ids[2..4])]),
+        "4" => row(&ids),
+        "2x3" => col(vec![row(&ids[0..3]), row(&ids[3..6])]),
+        "2x4" => col(vec![row(&ids[0..4]), row(&ids[4..8])]),
+        _ => return None,
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::ops::leaves;
+    fn ids(n: usize) -> Vec<SurfaceId> { (0..n).map(|i| SurfaceId(format!("s_{i}"))).collect() }
+
+    #[test]
+    fn all_presets_have_counts() {
+        for p in ["1","2lr","2tb","2+1","1+2","3","2x2","4","2x3","2x4"] {
+            assert!(slot_count(p).is_some(), "missing count for {p}");
+        }
+        assert!(slot_count("nope").is_none());
+    }
+
+    #[test]
+    fn build_uses_all_ids_in_order() {
+        for p in ["1","2lr","2tb","2+1","1+2","3","2x2","4","2x3","2x4"] {
+            let n = slot_count(p).unwrap();
+            let tree = build(p, &ids(n)).unwrap();
+            assert_eq!(leaves(&tree), ids(n), "preset {p} must place all ids in order");
+        }
+    }
+
+    #[test]
+    fn build_rejects_wrong_id_count() {
+        assert!(build("2x2", &ids(3)).is_none());
+        assert!(build("bogus", &ids(1)).is_none());
+    }
+
+    #[test]
+    fn grid_2x2_is_two_rows() {
+        let tree = build("2x2", &ids(4)).unwrap();
+        match tree {
+            LayoutNode::Split { orient: Orient::V, children, .. } => {
+                assert_eq!(children.len(), 2);
+                for r in &children {
+                    matches!(r, LayoutNode::Split { orient: Orient::H, .. });
+                }
+            }
+            _ => panic!("2x2 should be a vertical split of two horizontal rows"),
+        }
+    }
+}