feat: Add warning if skip_lines causes empty block

cmd/cmd.go

@@ -237,7 +237,7 @@ func fix(fixer *keepsorted.Fixer, filenames []string, modifiedLines []keepsorted
 			if warn.Lines.Start == warn.Lines.End {
 				log = log.Int("line", warn.Lines.Start)
 			} else {
-				log = log.Int("start", warn.Lines.Start).Int("end", warn.Lines.End)
+				log = log.Ints("[start,end]", []int{warn.Lines.Start, warn.Lines.End})
 			}
 			log.Msg(warn.Message)
 		}

goldens/skip_lines.err

@@ -0,0 +1,4 @@
+WRN block start is at or after end, possibly due to skip_lines [6+1,7-0] [start,end]=[6,7]
+WRN block start is at or after end, possibly due to skip_lines [10+0,11-1] [start,end]=[10,11]
+WRN block start is at or after end, possibly due to skip_lines [23+10,27-0] [start,end]=[23,27]
+exit status 1

goldens/skip_lines.in

@@ -1,7 +1,15 @@
-Skip lines with an empty block:
+Skip no lines with an empty block is ok:
+keep-sorted-test start skip_lines=0
+keep-sorted-test end
+
+Skip lines at start with an empty block:
 keep-sorted-test start skip_lines=1
 keep-sorted-test end
 
+Skip lines at end with an empty block:
+keep-sorted-test start skip_lines=-1
+keep-sorted-test end
+
 Skip two lines:
 // keep-sorted-test start skip_lines=2
 foo
@@ -11,7 +19,7 @@ b
 a
 // keep-sorted-test end
 
-Number of skipped lines is greater than block size, so block is ignored:
+Number of skipped lines is greater than block size, so skip_lines causes warning:
 // keep-sorted-test start skip_lines=10
 z
 y

goldens/skip_lines.out

@@ -1,7 +1,15 @@
-Skip lines with an empty block:
+Skip no lines with an empty block is ok:
+keep-sorted-test start skip_lines=0
+keep-sorted-test end
+
+Skip lines at start with an empty block:
 keep-sorted-test start skip_lines=1
 keep-sorted-test end
 
+Skip lines at end with an empty block:
+keep-sorted-test start skip_lines=-1
+keep-sorted-test end
+
 Skip two lines:
 // keep-sorted-test start skip_lines=2
 foo
@@ -11,7 +19,7 @@ b
 c
 // keep-sorted-test end
 
-Number of skipped lines is greater than block size, so block is ignored:
+Number of skipped lines is greater than block size, so skip_lines causes warning:
 // keep-sorted-test start skip_lines=10
 z
 y

keepsorted/block.go

@@ -15,6 +15,7 @@
 package keepsorted
 
 import (
+	"fmt"
 	"slices"
 	"strings"
 
@@ -116,6 +117,10 @@ func (f *Fixer) newBlocks(filename string, lines []string, offset int, include f
 			start.index += opts.startOffset()
 			endIndex += opts.endOffset()
 			if start.index >= endIndex {
+				warnings = append(warnings, finding(filename, start.index-opts.startOffset()+offset, endIndex-opts.endOffset()+offset,
+					fmt.Sprintf("block start is at or after end, possibly due to skip_lines [%d+%d,%d-%d]",
+						start.index-opts.startOffset()+offset, opts.startOffset(), endIndex-opts.endOffset()+offset, -opts.endOffset())),
+				)
 				continue
 			}