pdfgutter: never rewrite a page tree it cannot account for

parity/KNOWN.md

@@ -217,13 +217,62 @@ pixel, and a parity test that demanded it would be a test of pdfium rather
 than of this.
 
 What is compared instead is everything that decides: the aspect ratio, which
-pages get sampled, whether a given ink profile has a gutter, and the verdict
-a set of ratios and gutters adds up to. Both sides are handed the same
-profiles and have to agree exactly.
+pages get sampled, the reduction from pixels to a column profile, whether a
+given profile has a gutter, and the verdict a set of ratios and gutters adds
+up to. Both sides are handed the same pixels and the same profiles, and have
+to agree exactly.
 
-**Measured:** 72 cases, including profiles either side of the ink threshold by
-one ten-thousandth and profiles too narrow to hold the three bands the test
-looks at.
+The two renderers do have to be pointed at the same rectangle, which is what
+`-cropbox` is for — see the next note.
+
+**Measured:** 94 cases, including profiles either side of the ink threshold by
+one ten-thousandth, profiles too narrow to hold the three bands the test looks
+at, and greymaps whose heights do not divide evenly so that both sides have to
+round a fraction the same way.
+
+## pdfgutter — the page is the crop box, consistently
+
+**Python:** takes the aspect ratio from the media box and the ink profile
+from `page.render()`, which draws the **crop box**. On a page that carries
+both — most scanned books do — those are two different rectangles, so the
+shape is measured on one and the ink on the other. The bands the gutter is
+looked for in then fall somewhere other than the middle of what was
+rendered, and a real gutter is missed.
+
+**Rust:** the crop box is what a reader shows, so it is what everything
+measures: the ratio, the render (`pdftoppm -cropbox`), and where the cut
+goes. A page with no crop box is its media box, which is what the spec says
+and what most files are.
+
+**Why this is acceptable:** the original is not internally consistent, and on
+a wide sheet cropped to show only one of its two pages it gets the answer
+wrong in the destructive direction — the sheet still measures wide, so it is
+split again, down the middle of a page rather than between two. Measuring one
+rectangle throughout is the only version of this that is right.
+
+**Measured:** a wide sheet cropped to one page is no longer a split
+candidate, and a cropped spread's gutter lands where it is looked for. The
+arithmetic that both sides share is compared exactly, as above.
+
+## pdfgutter — a page tree that does not add up is refused
+
+**Python:** pdfium enumerates the pages and the question never comes up.
+
+**Rust:** the page tree is walked directly, and a walk is stricter than a
+renderer: a node missing its `/Type`, a `/Type` written as a reference, a kid
+that is a dictionary rather than a reference to one — each of those is a page
+a strict walk does not see and a reader does. So the number of pages found is
+checked against the number the file says it has, and a document where they
+disagree is refused with exit 2.
+
+**Why this is acceptable:** splitting replaces the page tree, and a page left
+out of the new tree is a page deleted. A file this does not fully understand
+is one it must not rewrite. Refusing costs a document that could perhaps have
+been handled; not refusing costs pages, silently, in the copy you keep.
+
+**Measured:** five shapes that a strict walk under-counts, each refused rather
+than written short. A document with nothing wide enough to cut is not
+rewritten at all, so every page comes through however the tree is built.
 
 ## pdfgutter — the thresholds are arguments, not environment variables
 

parity/cases/pdfgutter/cases.ndjson

@@ -70,3 +70,25 @@
 {"name": "two-of-three", "kind": "verdict", "ratios": [1.4, 1.4, 1.4], "gutters": [true, true, false]}
 {"name": "just-over-the-ratio", "kind": "verdict", "ratios": [1.1501, 1.1501, 1.1501, 1.1501], "gutters": [true, true, true, true]}
 {"name": "exactly-the-ratio", "kind": "verdict", "ratios": [1.15, 1.15, 1.15, 1.15], "gutters": [true, true, true, true]}
+{"name": "black-and-white-columns", "kind": "greymap", "width": 4, "height": 2, "pixels": [0, 255, 0, 255, 0, 255, 0, 255]}
+{"name": "either-side-of-the-threshold", "kind": "greymap", "width": 3, "height": 1, "pixels": [158, 159, 160]}
+{"name": "the-threshold-itself-everywhere", "kind": "greymap", "width": 5, "height": 4, "pixels": [160, 160, 160, 160, 160, 160, 160, 160, 160, 160, 160, 160, 160, 160, 160, 160, 160, 160, 160, 160]}
+{"name": "one-below-the-threshold-everywhere", "kind": "greymap", "width": 5, "height": 4, "pixels": [159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159, 159]}
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255, 25Line truncated
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+{"name": "random-grey-5", "kind": "greymap", "width": 8, "height": 5, "pixels": [178, 69, 89, 139, 210, 54, 174, 36, 127, 33, 8, 247, 247, 111, 250, 90, 229, 91, 227, 80, 193, 181, 107, 223, 229, 112, 94, 111, 69, 157, 229, 4, 138, 69, 242, 143, 44, 75, 149, 221]}
+{"name": "random-grey-6", "kind": "greymap", "width": 5, "height": 64, "pixels": [178, 251, 117, 200, 103, 113, 203, 92, 127, 92, 91, 109, 150, 65, 139, 74, 183, 154, 40, 170, 232, 224, 90, 80, 132, 108, 42, 222, 104, 100, 125, 204, 2, 6, 44, 248, 90, 146, 203, 153, 21, 67, 131, 219, 55, 75, 211, 220, 168, 191, 60, 7, 245, 145, 78, 143, 125, 180, 154, 225, 209, 229, 48, 234, 87, 82, 12, 115, 129, 109, 213, 55, 7, 231, 234, 226, 193, 34, 35, 136, 81, 233, 32, 230, 36, 173, 154, 227, 124, 171, 80, 232, 104, 96, 10, 128, 70, 187, 78, 119, 250, 229, 148, 155, 220, 105, 83, 230, 192, 163, 214, 60, 174, 173, 226, 202, 234, 70, 66, 250, 3, 213, 41, 16, 139, 5, 190, 8, 147, 58, 9, 139, 174, 237, 240, 189, 70, 152, 229, 233, 169, 124, 160, 168, 64, 242, 61, 172, 151, 113, 8, 187, 8, 216, 250, 27, 146, 0, 214, 237, 184, 82, 67, 207, 122, 135, 161, 64, 19, 1, 247, 254, 3, 238, 47, 26, 172, 29, 181, 20, 196, 118, 212, 61, 102, 95, 144, 143, 94, 109, 64, 118, 202, 54, 135, 236, 125, 83, 208, 120, 240, 245, 140, 232, 77, 86, 113, 132, 158, 85, 85, 217, 163, 64, 72, 101, 197, 165, 196, 148, 200, 175, 112, 225, 190, 205, 215, 13, 220, 60, 38, 59, 75, 188, 71, 32, 232, 166, 53, 6, 240, 236, 82, 212, 29, 247, 107, 140, 182, 188, 132, 147, 24, 214, 99, 151, 45, 144, 144, 54, 214, 233, 118, 158, 212, 131, 153, 208, 198, 91, 31, 62, 0, 108, 68, 162, 41, 227, 60, 247, 111, 67, 31, 181, 138, 70, 67, 183, 222, 63, 56, 56, 238, 137, 160, 126, 173, 203, 210, 134, 213, 74, 200, 147, 34, 254, 90, 241, 59, 222, 14, 202, 220, 252, 109, 32, 178, 83, 174, 23]}
+{"name": "random-grey-7", "kind": "greymap", "width": 5, "height": 64, "pixels": [237, 130, 80, 75, 116, 48, 246, 14, 244, 146, 93, 102, 14, 155, 255, 149, 103, 114, 123, 156, 222, 218, 216, 65, 16, 0, 132, 229, 67, 124, 89, 57, 213, 122, 83, 223, 7, 41, 221, 218, 68, 71, 193, 124, 23, 134, 104, 23, 158, 142, 108, 111, 213, 230, 221, 67, 107, 146, 252, 243, 219, 185, 127, 218, 83, 44, 24, 65, 1, 165, 217, 220, 189, 131, 9, 233, 73, 156, 219, 123, 98, 15, 29, 76, 246, 159, 8, 60, 151, 122, 165, 55, 148, 39, 113, 197, 134, 16, 112, 155, 9, 188, 142, 147, 19, 101, 240, 220, 47, 7, 9, 140, 93, 215, 200, 246, 165, 64, 9, 140, 231, 13, 169, 135, 88, 134, 114, 229, 157, 6, 168, 82, 18, 53, 192, 85, 187, 75, 212, 209, 106, 215, 225, 242, 152, 242, 201, 225, 240, 209, 26, 225, 238, 147, 124, 0, 133, 39, 206, 17, 90, 81, 19, 185, 110, 131, 177, 219, 29, 143, 194, 159, 227, 147, 190, 194, 201, 23, 75, 207, 99, 13, 14, 45, 99, 161, 73, 19, 132, 227, 36, 52, 228, 27, 174, 34, 165, 221, 14, 192, 26, 163, 169, 233, 230, 44, 45, 235, 219, 75, 65, 48, 148, 249, 44, 238, 79, 255, 220, 249, 250, 194, 33, 98, 118, 140, 76, 110, 252, 45, 64, 80, 248, 86, 154, 79, 87, 29, 251, 246, 56, 159, 42, 180, 122, 129, 73, 40, 156, 205, 76, 236, 58, 108, 80, 250, 97, 229, 236, 130, 23, 96, 60, 145, 68, 242, 202, 232, 3, 102, 238, 234, 61, 178, 6, 24, 75, 48, 242, 63, 101, 40, 213, 232, 92, 56, 83, 115, 53, 12, 37, 41, 43, 21, 67, 151, 211, 102, 71, 71, 135, 220, 179, 127, 16, 114, 93, 82, 47, 64, 109, 66, 241, 45, 196, 184, 127, 155, 206, 252]}
+{"name": "random-grey-8", "kind": "greymap", "width": 8, "height": 5, "pixels": [234, 23, 77, 84, 189, 212, 152, 21, 100, 47, 204, 255, 80, 21, 236, 208, 132, 213, 84, 81, 236, 76, 34, 121, 55, 100, 82, 85, 221, 50, 138, 15, 154, 136, 117, 41, 152, 225, 247, 94]}
+{"name": "random-grey-9", "kind": "greymap", "width": 5, "height": 64, "pixels": [46, 52, 170, 182, 133, 34, 95, 14, 143, 42, 127, 87, 126, 224, 170, 50, 131, 96, 231, 82, 67, 153, 28, 75, 164, 196, 4, 50, 104, 144, 222, 217, 236, 245, 36, 205, 231, 133, 156, 81, 126, 125, 253, 20, 146, 36, 6, 116, 243, 136, 74, 172, 28, 247, 188, 115, 111, 74, 217, 88, 71, 54, 230, 166, 236, 105, 220, 203, 236, 76, 221, 65, 120, 61, 200, 95, 98, 165, 105, 189, 38, 141, 76, 73, 130, 15, 168, 99, 223, 215, 161, 155, 176, 126, 137, 108, 122, 254, 158, 77, 238, 133, 96, 167, 8, 57, 87, 255, 231, 85, 186, 155, 55, 222, 127, 27, 54, 1, 223, 34, 12, 72, 76, 218, 73, 204, 169, 123, 172, 234, 160, 80, 211, 129, 74, 102, 158, 133, 20, 76, 96, 150, 163, 87, 102, 106, 47, 94, 189, 82, 95, 79, 31, 176, 55, 63, 247, 100, 191, 27, 130, 64, 32, 100, 63, 116, 154, 100, 89, 135, 129, 133, 231, 128, 175, 24, 95, 189, 70, 122, 13, 182, 218, 59, 109, 137, 184, 4, 238, 139, 178, 189, 187, 212, 73, 166, 67, 121, 66, 174, 130, 66, 111, 172, 80, 241, 190, 255, 164, 179, 78, 222, 134, 204, 253, 205, 30, 182, 60, 240, 134, 248, 74, 16, 52, 216, 42, 91, 216, 191, 198, 237, 90, 105, 11, 88, 249, 61, 245, 212, 57, 142, 191, 176, 199, 41, 103, 1, 167, 241, 87, 71, 125, 85, 82, 160, 165, 229, 145, 80, 34, 161, 195, 103, 144, 232, 105, 47, 210, 238, 111, 58, 147, 179, 42, 48, 140, 93, 78, 6, 202, 90, 146, 54, 222, 103, 222, 57, 232, 106, 224, 145, 192, 203, 154, 144, 39, 115, 107, 143, 255, 252, 21, 93, 6, 244, 223, 195, 149, 135, 117, 242, 69, 246, 130, 15, 76, 253, 111, 21]}

parity/golden/pdfgutter/cases.ndjson

@@ -70,3 +70,25 @@
 {"gutter_fraction": 0.6666666666666666, "kind": "verdict", "median_ratio": 1.4, "name": "two-of-three", "two_up": true}
 {"gutter_fraction": 1.0, "kind": "verdict", "median_ratio": 1.1501, "name": "just-over-the-ratio", "two_up": true}
 {"gutter_fraction": 1.0, "kind": "verdict", "median_ratio": 1.15, "name": "exactly-the-ratio", "two_up": false}
+{"kind": "greymap", "name": "black-and-white-columns", "profile": [1.0, 0.0, 1.0, 0.0]}
+{"kind": "greymap", "name": "either-side-of-the-threshold", "profile": [1.0, 1.0, 0.0]}
+{"kind": "greymap", "name": "the-threshold-itself-everywhere", "profile": [0.0, 0.0, 0.0, 0.0, 0.0]}
+{"kind": "greymap", "name": "one-below-the-threshold-everywhere", "profile": [1.0, 1.0, 1.0, 1.0, 1.0]}
+{"kind": "greymap", "name": "a-page-with-a-gutter", "profile": [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.35, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]}
+{"kind": "greymap", "name": "a-single-row", "profile": [1.0, 0.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0]}
+{"kind": "greymap", "name": "a-single-column", "profile": [0.5555555555555556]}
+{"kind": "greymap", "name": "awkward-7x3", "profile": [1.0, 1.0, 1.0, 1.0, 0.6666666666666666, 0.0, 0.0]}
+{"kind": "greymap", "name": "awkward-11x7", "profile": [1.0, 1.0, 1.0, 0.7142857142857143, 0.2857142857142857, 0.0, 0.42857142857142855, 1.0, 1.0, 1.0, 0.7142857142857143]}
+{"kind": "greymap", "name": "awkward-13x13", "profile": [1.0, 0.9230769230769231, 0.6153846153846154, 0.38461538461538464, 0.38461538461538464, 0.46153846153846156, 0.6923076923076923, 1.0, 0.8461538461538461, 0.6153846153846154, 0.38461538461538464, 0.3076923076923077, 0.46153846153846156]}
+{"kind": "greymap", "name": "awkward-3x101", "profile": [0.6534653465346535, 0.6534653465346535, 0.6534653465346535]}
+{"kind": "greymap", "name": "awkward-101x3", "profile": [1.0, 1.0, 1.0, 1.0, 0.6666666666666666, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 0.3333333333333333, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 0.3333333333333333, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 0.3333333333333333, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0, 0.3333333333333333, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0, 0.3333333333333333, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0, 0.3333333333333333, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0, 0.3333333333333333, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0, 0.6666666666666666, 1.0, 1.0, 1.0, 0.6666666666666666, 0.0, 0.0, 0.6666666666666666, 1.0, 1.0, 1.0, 0.6666666666666666, 0.0, 0.0, 0.6666666666666666, 1.0, 1.0, 1.0, 0.6666666666666666, 0.0, 0.0, 0.6666666666666666, 1.0, 1.0, 1.0, 0.6666666666666666, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0, 0.3333333333333333, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0]}
+{"kind": "greymap", "name": "random-grey-0", "profile": [0.5, 0.5]}
+{"kind": "greymap", "name": "random-grey-1", "profile": [0.625, 0.640625, 0.6875, 0.609375, 0.65625]}
+{"kind": "greymap", "name": "random-grey-2", "profile": [0.3333333333333333, 0.3333333333333333, 0.6666666666666666, 0.3333333333333333, 0.3333333333333333, 0.3333333333333333, 0.6666666666666666, 1.0, 0.0, 0.6666666666666666, 0.6666666666666666, 0.3333333333333333, 0.6666666666666666, 0.6666666666666666, 0.6666666666666666, 0.0, 0.6666666666666666, 0.3333333333333333, 0.6666666666666666, 0.3333333333333333, 0.6666666666666666, 0.6666666666666666, 0.6666666666666666, 0.6666666666666666, 0.6666666666666666, 0.0, 0.6666666666666666, 0.3333333333333333, 0.6666666666666666, 1.0, 0.6666666666666666, 0.6666666666666666, 0.3333333333333333, 0.6666666666666666, 0.3333333333333333, 0.3333333333333333, 1.0, 0.3333333333333333, 0.3333333333333333, 0.3333333333333333, 0.6666666666666666, 1.0, 0.3333333333333333, 0.6666666666666666, 0.6666666666666666, 1.0, 0.6666666666666666, 1.0, 1.0, 0.3333333333333333, 0.3333333333333333, 0.3333333333333333, 0.6666666666666666, 0.3333333333333333, 0.6666666666666666, 0.6666666666666666, 0.3333333333333333, 0.0, 0.6666666666666666, 0.6666666666666666, 0.3333333333333333, 0.3333333333333333, 0.6666666666666666, 0.6666666666666666]}
+{"kind": "greymap", "name": "random-grey-3", "profile": [0.3333333333333333, 0.6666666666666666, 0.3333333333333333, 1.0, 1.0, 0.3333333333333333, 1.0, 0.6666666666666666, 0.3333333333333333, 0.6666666666666666, 0.3333333333333333, 0.6666666666666666, 0.3333333333333333, 0.6666666666666666, 0.6666666666666666, 0.3333333333333333, 0.3333333333333333, 0.3333333333333333, 1.0, 0.6666666666666666, 0.6666666666666666, 1.0, 0.6666666666666666, 0.3333333333333333, 0.3333333333333333, 0.6666666666666666, 0.6666666666666666, 0.6666666666666666, 1.0, 0.6666666666666666, 0.0, 0.6666666666666666, 0.3333333333333333, 0.3333333333333333, 1.0, 1.0, 0.3333333333333333, 0.3333333333333333, 0.3333333333333333, 0.6666666666666666, 0.6666666666666666, 0.6666666666666666, 0.3333333333333333, 1.0, 0.3333333333333333, 0.6666666666666666, 0.3333333333333333, 0.6666666666666666, 0.3333333333333333, 0.3333333333333333, 0.3333333333333333, 0.6666666666666666, 0.6666666666666666, 1.0, 0.3333333333333333, 0.3333333333333333, 1.0, 0.6666666666666666, 0.6666666666666666, 0.6666666666666666, 1.0, 1.0, 0.6666666666666666, 0.6666666666666666]}
+{"kind": "greymap", "name": "random-grey-4", "profile": [0.0, 0.5]}
+{"kind": "greymap", "name": "random-grey-5", "profile": [0.4, 1.0, 0.6, 0.8, 0.4, 0.8, 0.4, 0.6]}
+{"kind": "greymap", "name": "random-grey-6", "profile": [0.59375, 0.546875, 0.625, 0.5, 0.703125]}
+{"kind": "greymap", "name": "random-grey-7", "profile": [0.671875, 0.671875, 0.609375, 0.625, 0.671875]}
+{"kind": "greymap", "name": "random-grey-8", "profile": [0.8, 0.8, 0.8, 0.8, 0.4, 0.6, 0.6, 0.8]}
+{"kind": "greymap", "name": "random-grey-9", "profile": [0.671875, 0.671875, 0.640625, 0.484375, 0.703125]}

parity/oracle/pdfgutter.py

@@ -21,12 +21,19 @@ import numpy as np
 _MIN_RATIO = 1.15
 _MIN_GUTTER_FRAC = 0.6
 _TEXT_COL_INK = 0.02
+_INK_THRESHOLD = 160
 
 
 def _ratio(width: float, height: float) -> float:
     return width / height if height else 0.0
 
 
+def _col_ink_profile(pixels: list[int], width: int, height: int) -> list[float]:
+    """The reduction the renderer's output went through, minus the render."""
+    arr = np.asarray(pixels, dtype=np.uint8).reshape(height, width)
+    return [float(x) for x in (arr < _INK_THRESHOLD).mean(axis=0)]
+
+
 def _has_gutter(ink: np.ndarray) -> bool:
     w = len(ink)
     center = ink[int(0.46 * w):int(0.54 * w)]
@@ -67,6 +74,10 @@ def answer(case: dict) -> dict:
         return {"gutter": _has_gutter(np.asarray(case["ink"], dtype=float))}
     if kind == "sample":
         return {"indices": _sample_indices(case["pages"])}
+    if kind == "greymap":
+        return {
+            "profile": _col_ink_profile(case["pixels"], case["width"], case["height"])
+        }
     if kind == "verdict":
         return _verdict(case["ratios"], case["gutters"])
     raise SystemExit(f"no such case: {kind}")

parity/tests/pdfgutter.rs

@@ -76,6 +76,28 @@ fn every_answer_matches_python() {
                     note("indices", format!("{ours:?}"), &theirs["indices"]);
                 }
             }
+            "greymap" => {
+                let width = case["width"].as_u64().expect("a width") as usize;
+                let height = case["height"].as_u64().expect("a height") as usize;
+                let pixels: Vec<u8> = case["pixels"]
+                    .as_array()
+                    .expect("an array")
+                    .iter()
+                    .map(|value| value.as_u64().expect("a number") as u8)
+                    .collect();
+
+                // The renderer is not the thing under test; the reduction it
+                // feeds is. Both sides are handed the same pixels.
+                let mut greymap = format!("P5\n{width} {height}\n255\n").into_bytes();
+                greymap.extend_from_slice(&pixels);
+                let ours = pdfgutter::ink::profile(&greymap, pdfgutter::INK_THRESHOLD)
+                    .expect("a readable greymap");
+
+                let theirs_profile = floats(&theirs["profile"]);
+                if ours != theirs_profile {
+                    note("profile", format!("{ours:?}"), &theirs["profile"]);
+                }
+            }
             "verdict" => {
                 let ratios = floats(&case["ratios"]);
                 let gutters: Vec<bool> = case["gutters"]
@@ -124,7 +146,7 @@ fn every_answer_matches_python() {
 #[test]
 fn every_kind_of_case_is_present() {
     let cases = cases();
-    for kind in ["ratio", "gutter", "sample", "verdict"] {
+    for kind in ["ratio", "gutter", "sample", "verdict", "greymap"] {
         assert!(
             cases.iter().any(|case| case["kind"] == kind),
             "no {kind} case"