Templater: naive implementation of shortest prefix highlight for ids

This creates a templater function `short_underscore_prefix` for commit and
change ids. It is similar to `short` function, but shows one fewer hexadecimal
digit and inserts an underscore after the shortest unique prefix.

Highlighting with an underline and perhaps color/bold will be in a follow-up
PR.

The implementation is quadratic, a simple comparison of each id with every
other id. It is replaced in a subsequent commit. The problem with it is that,
while it works fine for a `jj`-sized repo, it becomes is painfully slow with a
repo the size of git/git. 

Still, this naive implemenation is included here since it's simple, and could
be used as a reference implementation. 

The `shortest_unique_prefix_length` function goes into `repo.rs` since that's
convenient for follow-up commits in this PR to have nicer diffs.

lib/src/repo.rs

@@ -24,7 +24,7 @@ use once_cell::sync::OnceCell;
 use thiserror::Error;
 
 use self::dirty_cell::DirtyCell;
-use crate::backend::{Backend, BackendError, BackendResult, ChangeId, CommitId, TreeId};
+use crate::backend::{Backend, BackendError, BackendResult, ChangeId, CommitId, ObjectId, TreeId};
 use crate::commit::Commit;
 use crate::commit_builder::CommitBuilder;
 use crate::dag_walk::topo_order_reverse;
@@ -37,6 +37,7 @@ use crate::op_store::{
     BranchTarget, OpStore, OperationId, OperationMetadata, RefTarget, WorkspaceId,
 };
 use crate::operation::Operation;
+use crate::revset::RevsetExpression;
 use crate::rewrite::DescendantRebaser;
 use crate::settings::{RepoSettings, UserSettings};
 use crate::simple_op_heads_store::SimpleOpHeadsStore;
@@ -241,6 +242,40 @@ impl ReadonlyRepo {
         })
     }
 
+    // An interface for testing this functionality directly is constructed in
+    // a follow-up commit.
+    pub fn shortest_unique_prefix_length(&self, target_id_hex: &str) -> usize {
+        let all_visible_revisions = RevsetExpression::all()
+            .evaluate(self.as_repo_ref(), None)
+            .unwrap();
+        let change_hex_iter = all_visible_revisions
+            .iter()
+            .map(|index_entry| index_entry.change_id().hex());
+        // We need to account for rewritten commits as well
+        let index = self.as_repo_ref().index();
+        let commit_hex_iter = index
+            .iter()
+            .map(|index_entry| index_entry.commit_id().hex());
+
+        let target_id_hex = target_id_hex.as_bytes();
+        itertools::chain(change_hex_iter, commit_hex_iter)
+            .filter_map(|id_hex| {
+                let id_hex = id_hex.as_bytes();
+                let common_len = target_id_hex
+                    .iter()
+                    .zip(id_hex.iter())
+                    .take_while(|(a, b)| a == b)
+                    .count();
+                if common_len == target_id_hex.len() && common_len == id_hex.len() {
+                    None // Target id matched itself
+                } else {
+                    Some(common_len + 1)
+                }
+            })
+            .max()
+            .unwrap_or(0)
+    }
+
     pub fn store(&self) -> &Arc<Store> {
         &self.store
     }

src/template_parser.rs

@@ -23,11 +23,12 @@ use pest_derive::Parser;
 use crate::formatter::PlainTextFormatter;
 use crate::templater::{
     AuthorProperty, BranchProperty, CommitOrChangeId, CommitOrChangeIdKeyword,
-    CommitOrChangeIdShort, CommitterProperty, ConditionalTemplate, ConflictProperty,
-    ConstantTemplateProperty, DescriptionProperty, DivergentProperty, DynamicLabelTemplate,
-    EmptyProperty, GitRefsProperty, IsGitHeadProperty, IsWorkingCopyProperty, LabelTemplate,
-    ListTemplate, LiteralTemplate, SignatureTimestamp, StringPropertyTemplate, TagProperty,
-    Template, TemplateFunction, TemplateProperty, WorkingCopiesProperty,
+    CommitOrChangeIdShort, CommitOrChangeIdShortPrefixAndBrackets, CommitterProperty,
+    ConditionalTemplate, ConflictProperty, ConstantTemplateProperty, DescriptionProperty,
+    DivergentProperty, DynamicLabelTemplate, EmptyProperty, GitRefsProperty, IsGitHeadProperty,
+    IsWorkingCopyProperty, LabelTemplate, ListTemplate, LiteralTemplate, SignatureTimestamp,
+    StringPropertyTemplate, TagProperty, Template, TemplateFunction, TemplateProperty,
+    WorkingCopiesProperty,
 };
 use crate::time_util;
 
@@ -215,6 +216,9 @@ fn parse_commit_or_chain_id_method<'a>(
 
     let this_function = match name.as_str() {
         "short" => Property::String(Box::new(CommitOrChangeIdShort { repo })),
+        "short_prefix_and_brackets" => {
+            Property::String(Box::new(CommitOrChangeIdShortPrefixAndBrackets { repo }))
+        }
         name => panic!("no such commit ID method: {name}"),
     };
     let chain_method = inner.last().unwrap();

src/templater.rs

@@ -444,6 +444,25 @@ impl CommitOrChangeIdKeyword {
     pub fn short_format(commit_or_change_id: CommitOrChangeId) -> String {
         commit_or_change_id.hex()[..12].to_string()
     }
+    pub fn short_prefix_and_brackets_format(
+        commit_or_change_id: CommitOrChangeId,
+        repo: RepoRef,
+    ) -> String {
+        highlight_shortest_prefix(&commit_or_change_id.hex(), 12, repo)
+    }
+}
+
+fn highlight_shortest_prefix(hex: &str, total_len: usize, repo: RepoRef) -> String {
+    let prefix_len = repo.base_repo().shortest_unique_prefix_length(hex);
+    if prefix_len < total_len - 1 {
+        format!(
+            "{}[{}]",
+            &hex[0..prefix_len],
+            &hex[prefix_len..total_len - 1]
+        )
+    } else {
+        hex[0..total_len].to_string()
+    }
 }
 
 pub struct CommitOrChangeIdShort<'a> {
@@ -456,6 +475,16 @@ impl TemplateProperty<CommitOrChangeId, String> for CommitOrChangeIdShort<'_> {
     }
 }
 
+pub struct CommitOrChangeIdShortPrefixAndBrackets<'a> {
+    pub repo: RepoRef<'a>,
+}
+
+impl TemplateProperty<CommitOrChangeId, String> for CommitOrChangeIdShortPrefixAndBrackets<'_> {
+    fn extract(&self, context: &CommitOrChangeId) -> String {
+        CommitOrChangeIdKeyword::short_prefix_and_brackets_format(context.clone(), self.repo)
+    }
+}
+
 impl TemplateProperty<Commit, CommitOrChangeId> for CommitOrChangeIdKeyword {
     fn extract(&self, context: &Commit) -> CommitOrChangeId {
         match &self.0 {

tests/test_log_command.rs

@@ -287,6 +287,113 @@ fn test_log_with_or_without_diff() {
     "###);
 }
 
+#[test]
+fn test_log_prefix_highlight() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    let prefix_format = r#"
+      "Change " change_id.short_prefix_and_brackets() " " description.first_line()
+      " " commit_id.short_prefix_and_brackets() " " branches
+    "#;
+
+    std::fs::write(repo_path.join("file"), "original file\n").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["describe", "-m", "initial"]);
+    test_env.jj_cmd_success(&repo_path, &["branch", "c", "original"]);
+    insta::assert_snapshot!(
+        test_env.jj_cmd_success(&repo_path, &["log", "-r", "original", "-T", prefix_format]),
+        @r###"
+    @ Change 9[a45c67d3e9] initial b[a1a30916d2] original
+    ~ 
+    "###
+    );
+    for i in 1..50 {
+        test_env.jj_cmd_success(&repo_path, &["new", "-m", &format!("commit{i}")]);
+        std::fs::write(repo_path.join("file"), format!("file {i}\n")).unwrap();
+    }
+    insta::assert_snapshot!(
+        test_env.jj_cmd_success(&repo_path, &["log", "-r", "original", "-T", prefix_format]),
+        @r###"
+    o Change 9a4[5c67d3e9] initial ba1[a30916d2] original
+    ~ 
+    "###
+    );
+    insta::assert_snapshot!(
+        test_env.jj_cmd_success(&repo_path, &["log", "-r", "@-----------..@", "-T", prefix_format]),
+        @r###"
+    @ Change 4c9[32da8013] commit49 d8[3437a2cef] 
+    o Change 0d[58f15eaba] commit48 f3[abb4ea0ac] 
+    o Change fc[e6c2c5912] commit47 38e[891bea27] 
+    o Change d5[1defcac30] commit46 1c[04d947707] 
+    o Change 4f[13b1391d6] commit45 747[24ae22b1] 
+    o Change 6a[de2950a04] commit44 c7a[a67cf7bb] 
+    o Change 06c[482e452d] commit43 8e[c99dfcb6c] 
+    o Change 392[beeb018e] commit42 8f0[e60411b7] 
+    o Change a1[b73d3ff91] commit41 71[d6937a66c] 
+    o Change 708[8f461291] commit40 db[572049026] 
+    o Change c49[f7f006c7] commit39 d94[54fec8a6] 
+    ~ 
+    "###
+    );
+}
+
+#[test]
+fn test_log_prefix_highlight_counts_hidden_commits() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    let prefix_format = r#"
+      "Change " change_id.short_prefix_and_brackets() " " description.first_line()
+      " " commit_id.short_prefix_and_brackets() " " branches
+    "#;
+
+    std::fs::write(repo_path.join("file"), "original file\n").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["describe", "-m", "initial"]);
+    test_env.jj_cmd_success(&repo_path, &["branch", "c", "original"]);
+    insta::assert_snapshot!(
+        test_env.jj_cmd_success(&repo_path, &["log", "-r", "all()", "-T", prefix_format]),
+        @r###"
+    @ Change 9[a45c67d3e9] initial b[a1a30916d2] original
+    o Change 000000000000 (no description set) 000000000000 
+    "###
+    );
+    for i in 1..100 {
+        test_env.jj_cmd_success(&repo_path, &["describe", "-m", &format!("commit{i}")]);
+        std::fs::write(repo_path.join("file"), format!("file {i}\n")).unwrap();
+    }
+    // The first commit still exists. Its unique prefix became longer.
+    insta::assert_snapshot!(
+        test_env.jj_cmd_success(&repo_path, &["log", "-r", "ba1", "-T", prefix_format]),
+        @r###"
+    o Change 9a4[5c67d3e9] initial ba1[a30916d2] 
+    ~ 
+    "###
+    );
+    // The first commit is no longer visible
+    insta::assert_snapshot!(
+        test_env.jj_cmd_success(&repo_path, &["log", "-r", "all()", "-T", prefix_format]),
+        @r###"
+    @ Change 9a4[5c67d3e9] commit99 de[3177d2acf] original
+    o Change 000000000000 (no description set) 000000000000 
+    "###
+    );
+    insta::assert_snapshot!(
+        test_env.jj_cmd_failure(&repo_path, &["log", "-r", "d", "-T", prefix_format]),
+        @r###"
+    Error: Commit or change id prefix "d" is ambiguous
+    "###
+    );
+    insta::assert_snapshot!(
+        test_env.jj_cmd_success(&repo_path, &["log", "-r", "de", "-T", prefix_format]),
+        @r###"
+    @ Change 9a4[5c67d3e9] commit99 de[3177d2acf] original
+    ~ 
+    "###
+    );
+}
+
 #[test]
 fn test_log_divergence() {
     let test_env = TestEnvironment::default();