From 24a84671ace373ae331fa83a1ff484990f4dff0e Mon Sep 17 00:00:00 2001
From: Lukasz Kasprzak <lukas@labunix.xyz>
Date: Sat, 12 Sep 2026 12:58:14 +0200
Subject: krino: planning — chains, placeholders, conflicts, JSON
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---
 internal/dup/dup.go      | 41 +++++++++++++++++++++++++++++++++++++++++
 internal/dup/dup_test.go | 23 +++++++++++++++++++++++
 2 files changed, 64 insertions(+)

(limited to 'internal/dup')

diff --git a/internal/dup/dup.go b/internal/dup/dup.go
index 502ef31..e3c7424 100644
--- a/internal/dup/dup.go
+++ b/internal/dup/dup.go
@@ -381,6 +381,47 @@ func readAt(f *os.File, off int64) ([]byte, error) {
 	return buf[:n], nil
 }
 
+// SameContent reports whether a and b hold identical content: a stat and
+// size check first, then the same partial/full hash comparison Lookup uses
+// for scanned candidates. Neither file needs to have been scanned or
+// indexed; this is the one place content identity is decided, so callers
+// outside this package must not hash a second way.
+func SameContent(a, b string) (bool, error) {
+	ai, err := os.Stat(a)
+	if err != nil {
+		return false, err
+	}
+	bi, err := os.Stat(b)
+	if err != nil {
+		return false, err
+	}
+	if ai.Size() != bi.Size() {
+		return false, nil
+	}
+
+	aPartial, err := computePartialHash(a)
+	if err != nil {
+		return false, err
+	}
+	bPartial, err := computePartialHash(b)
+	if err != nil {
+		return false, err
+	}
+	if aPartial != bPartial {
+		return false, nil
+	}
+
+	aFull, err := computeFullHash(a)
+	if err != nil {
+		return false, err
+	}
+	bFull, err := computeFullHash(b)
+	if err != nil {
+		return false, err
+	}
+	return aFull == bFull, nil
+}
+
 // computeFullHash hashes the whole file.
 func computeFullHash(path string) ([sha256.Size]byte, error) {
 	f, err := os.Open(path)
diff --git a/internal/dup/dup_test.go b/internal/dup/dup_test.go
index fb4e64d..5d2621c 100644
--- a/internal/dup/dup_test.go
+++ b/internal/dup/dup_test.go
@@ -262,6 +262,29 @@ func TestLookupFailsWhenSubjectUnreadable(t *testing.T) {
 	}
 }
 
+// TestSameContentEqualSizeDifferentContent: two files of identical size but
+// different bytes must not be reported as the same content — the partial
+// hash (not just the size check) has to separate them.
+func TestSameContentEqualSizeDifferentContent(t *testing.T) {
+	d := t.TempDir()
+	a := filepath.Join(d, "a.bin")
+	b := filepath.Join(d, "b.bin")
+	one := []byte("acme-invoice-01")
+	two := []byte("acme-invoice-02")
+	if len(one) != len(two) {
+		t.Fatal("fixture bug: files must be the same size")
+	}
+	if err := os.WriteFile(a, one, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(b, two, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if ok, err := SameContent(a, b); err != nil || ok {
+		t.Errorf("SameContent(a, b) = %v, %v; want false, nil", ok, err)
+	}
+}
+
 // fakeDirEntry is an fs.DirEntry whose Info() returns a canned result, for
 // exercising addEntry's Info()-failure handling directly (A3) — a real
 // filepath.WalkDir gives no hook to inject a stat failure deterministically
-- 
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