From 5e22f9b373f32c3af7cee5f0f339482c207ed6a6 Mon Sep 17 00:00:00 2001
From: nihilist-narwhal <240798405+nihilist-narwhal@users.noreply.github.com>
Date: Sat, 10 Oct 2026 03:40:56 -0500
Subject: [PATCH] Sort: case-sensitive order puts lowercase before uppercase

With Case sensitive checked, Sort compared text by code point, so every
uppercase word came before every lowercase one (A, B, a, b). Excel
compares ignoring case first and only then puts lowercase before
uppercase: a, A, b, B.
---
 crates/engine/src/cmd/data.rs | 8 +++++++-
 crates/engine/src/tests.rs    | 9 +++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/crates/engine/src/cmd/data.rs b/crates/engine/src/cmd/data.rs
index fd50afbf2..94733071c 100644
--- a/crates/engine/src/cmd/data.rs
+++ b/crates/engine/src/cmd/data.rs
@@ -248,7 +248,7 @@ fn do_sort(s: &mut Session, r: RangeRef, header: bool, keys: Vec<SortKey>, by_co
                     let pos = |v: &Value| v.as_text().and_then(|t| list.iter().position(|x| x.eq_ignore_ascii_case(t))).unwrap_or(usize::MAX);
                     pos(va).cmp(&pos(vb))
                 } else if case && va.is_text() && vb.is_text() {
-                    va.as_text().unwrap_or("").cmp(vb.as_text().unwrap_or(""))
+                    sort_compare(va, vb).then_with(|| case_order(va.as_text().unwrap_or(""), vb.as_text().unwrap_or("")))
                 } else {
                     sort_compare(va, vb)
                 };
@@ -653,6 +653,12 @@ pub fn filter_values(wb: &Workbook, sheet: usize, col: u32) -> Vec<(String, bool
     seen.into_iter().map(|((_, t), on)| (t, on)).collect()
 }
 
+/// Case-sensitive tie-break for text that is equal ignoring case: as in Excel, lowercase sorts
+/// before uppercase at the first letter that differs in case.
+fn case_order(a: &str, b: &str) -> Ordering {
+    a.chars().zip(b.chars()).find(|(x, y)| x != y).map_or(Ordering::Equal, |(x, y)| y.is_lowercase().cmp(&x.is_lowercase()))
+}
+
 fn sort_key(v: &Value) -> (u8, i64, String) {
     match v {
         Value::Number(n) => (0, (n * 1e6) as i64, String::new()),
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 6bd03d55e..92ca38665 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -606,6 +606,15 @@ fn hidden_formulas_stay_hidden_on_protected_sheets() {
     assert_eq!(r["results"][0]["value"], "7");
 }
 
+#[test]
+fn case_sensitive_sort_puts_lowercase_first() {
+    let mut s = s();
+    s.execute("range.setValues", json!({"range": "A1", "values": [["b"], ["B"], ["Ab"], ["a"], ["aB"], ["A"]]})).unwrap();
+    s.execute("data.sort", json!({"range": "A1:A6", "header": false, "matchCase": true, "keys": [{"column": "A"}]})).unwrap();
+    let col: Vec<Value> = (1..=6).map(|r| v(&s, &format!("A{r}"))).collect();
+    assert_eq!(col, ["a", "A", "aB", "Ab", "b", "B"].map(Value::from));
+}
+
 #[test]
 fn formatting_and_styles() {
     let mut s = s();