the suffix search continues instead of starting again at _1

Every file renamed onto one name probed stem_1, stem_2, ... from the
beginning, so N files cost N^2/2 Exists calls - a test here counts 1890
of them for 60 files. The search now continues from the highest suffix
already tried for that stem.

Within one plan that is the same answer: the taken set only grows while
a plan is built and the disk is not being written to, so a suffix taken
once stays taken. Proved rather than argued - with same_1 and same_3
already on disk and same_2 free, both versions put a file in the gap,
and the two plans are byte-identical.

  1500 files renamed to one name: 2.63s -> 0.05s

1 files changed, 53 insertions, 1 deletions

diff --git a/internal/plan/conflict_test.go b/internal/plan/conflict_test.go
index ba415d5..ba111f8 100644
--- a/internal/plan/conflict_test.go
+++ b/internal/plan/conflict_test.go
@@ -3,6 +3,7 @@
 package plan
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 	"testing"
@@ -232,7 +233,7 @@ func TestSuffixedCapsAttempts(t *testing.T) {
 func TestOverwriteNeverDisplacesADirectory(t *testing.T) {
 	dir := "/home/x/Documents/Invoices"
 	d := fakeDisk{exists: map[string]bool{dir: true}, dirs: map[string]bool{dir: true}}
-	_, skip, displaces := resolveConflict(Move, config.ConflictOverwrite, "/r/Invoices", dir, d, claimed{})
+	_, skip, displaces := resolveConflict(Move, config.ConflictOverwrite, "/r/Invoices", dir, d, newClaimed())
 	if displaces != "" || skip != "target is not a regular file" {
 		t.Errorf("skip %q displaces %q; want skipped, nothing displaced", skip, displaces)
 	}
@@ -252,3 +253,54 @@ func TestOverwriteNeverDisplacesAnotherScannedFile(t *testing.T) {
 		t.Errorf("Displaces %q Dst %q Skip %q; want no displacement and /r/b_1.pdf", s.Displaces, s.Dst, s.Skip)
 	}
 }
+
+// countingDisk counts Exists calls, so a test can pin how much probing a
+// plan does rather than only what it produces.
+type countingDisk struct {
+	fakeDisk
+	calls int
+}
+
+func (d *countingDisk) Exists(p string) bool {
+	d.calls++
+	return d.fakeDisk.Exists(p)
+}
+
+// TestSuffixedDoesNotRescanFromOne: every file renamed to one name probes
+// stem_1, stem_2, ... for a free suffix. Starting each file's search at 1
+// makes N files into N^2/2 Exists calls - 4000 files were eight million of
+// them - so the search continues from the highest suffix already tried for
+// that name. Within one plan that is the same answer: the taken set only
+// grows while a plan is built, so a suffix taken once stays taken.
+func TestSuffixedDoesNotRescanFromOne(t *testing.T) {
+	const n = 60
+	in := make([]Input, 0, n)
+	for i := 0; i < n; i++ {
+		in = append(in, Input{File: file("/r", fmt.Sprintf("f%02d.pdf", i)), Rules: []RuleMatch{
+			{Name: "one", Actions: []config.Action{act(config.Move, "Work"), act(config.Rename, "same.pdf")}}}})
+	}
+	d := &countingDisk{fakeDisk: fakeDisk{exists: map[string]bool{}}}
+	chains := Build("/r", in, time.Now(), d, NewClaims())
+
+	// Every file must still land on its own name, the lowest free one.
+	seen := map[string]bool{}
+	for _, c := range chains {
+		last := c.Steps[len(c.Steps)-1]
+		if last.Dst == "" {
+			t.Fatalf("%s was skipped: %q", c.File.Rel, last.Skip)
+		}
+		if seen[last.Dst] {
+			t.Errorf("two files planned onto %s", last.Dst)
+		}
+		seen[last.Dst] = true
+	}
+	if len(seen) != n {
+		t.Errorf("%d distinct destinations for %d files", len(seen), n)
+	}
+	// Quadratic probing would be about n*n/2 = 1800 here; linear is a few
+	// per file. The bound is loose on purpose - it must fail on n^2 and
+	// pass on anything sane.
+	if d.calls > 6*n {
+		t.Errorf("%d Exists calls for %d files: the suffix search is rescanning from _1", d.calls, n)
+	}
+}