SkPDF: pull out SkPDFMakeCIDGlyphWidthsArray.cpp

src/pdf/SkPDFMakeCIDGlyphWidthsArray.cpp

Index: src/pdf/SkPDFMakeCIDGlyphWidthsArray.cpp
diff --git a/src/pdf/SkPDFMakeCIDGlyphWidthsArray.cpp b/src/pdf/SkPDFMakeCIDGlyphWidthsArray.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..e1bb0d27859409fcd10c54f38f949f668989c425
--- /dev/null
+++ b/src/pdf/SkPDFMakeCIDGlyphWidthsArray.cpp
@@ -0,0 +1,262 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkBitSet.h"
+#include "SkPDFMakeCIDGlyphWidthsArray.h"
+#include "SkPaint.h"
+#include "SkGlyphCache.h"
+
+// TODO(halcanary): Write unit tests for SkPDFMakeCIDGlyphWidthsArray().
+
+// TODO(halcanary): The logic in this file originated in several
+// disparate places.  I feel sure that someone could simplify this
+// down to a single easy-to-read function.
+
+namespace {
+
+struct AdvanceMetric {
+    enum MetricType {
+        kDefault,  // Default advance: fAdvance.count = 1
+        kRange,    // Advances for a range: fAdvance.count = fEndID-fStartID
+        kRun       // fStartID-fEndID have same advance: fAdvance.count = 1
+    };
+    MetricType fType;
+    uint16_t fStartId;
+    uint16_t fEndId;
+    SkTDArray<int16_t> fAdvance;
+    AdvanceMetric(uint16_t startId) : fStartId(startId) {}
+    AdvanceMetric(AdvanceMetric&&) = default;
+    AdvanceMetric& operator=(AdvanceMetric&& other) = default;
+    AdvanceMetric(const AdvanceMetric&) = delete;
+    AdvanceMetric& operator=(const AdvanceMetric&) = delete;
+};
+const int16_t kInvalidAdvance = SK_MinS16;
+const int16_t kDontCareAdvance = SK_MinS16 + 1;
+} // namespace
+
+// scale from em-units to base-1000, returning as a SkScalar
+static SkScalar from_font_units(SkScalar scaled, uint16_t emSize) {
+    if (emSize == 1000) {
+        return scaled;
+    } else {
+        return SkScalarMulDiv(scaled, 1000, emSize);
+    }
+}
+
+static SkScalar scale_from_font_units(int16_t val, uint16_t emSize) {
+    return from_font_units(SkIntToScalar(val), emSize);
+}
+
+static void strip_uninteresting_trailing_advances_from_range(
+        AdvanceMetric* range) {
+    SkASSERT(range);
+
+    int expectedAdvanceCount = range->fEndId - range->fStartId + 1;
+    if (range->fAdvance.count() < expectedAdvanceCount) {
+        return;
+    }
+
+    for (int i = expectedAdvanceCount - 1; i >= 0; --i) {
+        if (range->fAdvance[i] != kDontCareAdvance &&
+            range->fAdvance[i] != kInvalidAdvance &&
+            range->fAdvance[i] != 0) {
+            range->fEndId = range->fStartId + i;
+            break;
+        }
+    }
+}
+
+static void zero_wildcards_in_range(AdvanceMetric* range) {
+    SkASSERT(range);
+    if (range->fType != AdvanceMetric::kRange) {
+        return;
+    }
+    SkASSERT(range->fAdvance.count() == range->fEndId - range->fStartId + 1);
+
+    // Zero out wildcards.
+    for (int i = 0; i < range->fAdvance.count(); ++i) {
+        if (range->fAdvance[i] == kDontCareAdvance) {
+            range->fAdvance[i] = 0;
+        }
+    }
+}
+
+static void finish_range(
+        AdvanceMetric* range,
+        int endId,
+        AdvanceMetric::MetricType type) {
+    range->fEndId = endId;
+    range->fType = type;
+    strip_uninteresting_trailing_advances_from_range(range);
+    int newLength;
+    if (type == AdvanceMetric::kRange) {
+        newLength = range->fEndId - range->fStartId + 1;
+    } else {
+        if (range->fEndId == range->fStartId) {
+            range->fType = AdvanceMetric::kRange;
+        }
+        newLength = 1;
+    }
+    SkASSERT(range->fAdvance.count() >= newLength);
+    range->fAdvance.setCount(newLength);
+    zero_wildcards_in_range(range);
+}
+
+static void compose_advance_data(const AdvanceMetric& range,
+                                 uint16_t emSize,
+                                 int16_t* defaultAdvance,
+                                 SkPDFArray* result) {
+    switch (range.fType) {
+        case AdvanceMetric::kDefault: {
+            SkASSERT(range.fAdvance.count() == 1);
+            *defaultAdvance = range.fAdvance[0];
+            break;
+        }
+        case AdvanceMetric::kRange: {
+            auto advanceArray = sk_make_sp<SkPDFArray>();
+            for (int j = 0; j < range.fAdvance.count(); j++)
+                advanceArray->appendScalar(
+                        scale_from_font_units(range.fAdvance[j], emSize));
+            result->appendInt(range.fStartId);
+            result->appendObject(std::move(advanceArray));
+            break;
+        }
+        case AdvanceMetric::kRun: {
+            SkASSERT(range.fAdvance.count() == 1);
+            result->appendInt(range.fStartId);
+            result->appendInt(range.fEndId);
+            result->appendScalar(
+                    scale_from_font_units(range.fAdvance[0], emSize));
+            break;
+        }
+    }
+}
+
+/** Retrieve advance data for glyphs. Used by the PDF backend. */
+// TODO(halcanary): this function is complex enough to need its logic
+// tested with unit tests.
+sk_sp<SkPDFArray> SkPDFMakeCIDGlyphWidthsArray(SkGlyphCache* cache,
+                                               const SkBitSet* subset,
+                                               uint16_t emSize,
+                                               int16_t* defaultAdvance) {
+    // Assuming that on average, the ASCII representation of an advance plus
+    // a space is 8 characters and the ASCII representation of a glyph id is 3
+    // characters, then the following cut offs for using different range types
+    // apply:
+    // The cost of stopping and starting the range is 7 characers
+    //  a. Removing 4 0's or don't care's is a win
+    // The cost of stopping and starting the range plus a run is 22
+    // characters
+    //  b. Removing 3 repeating advances is a win
+    //  c. Removing 2 repeating advances and 3 don't cares is a win
+    // When not currently in a range the cost of a run over a range is 16
+    // characaters, so:
+    //  d. Removing a leading 0/don't cares is a win because it is omitted
+    //  e. Removing 2 repeating advances is a win
+
+    auto result = sk_make_sp<SkPDFArray>();
+    int num_glyphs = SkToInt(cache->getGlyphCount());
+
+    bool prevRange = false;
+
+    int16_t lastAdvance = kInvalidAdvance;
+    int repeatedAdvances = 0;
+    int wildCardsInRun = 0;
+    int trailingWildCards = 0;
+
+    // Limit the loop count to glyph id ranges provided.
+    int lastIndex = num_glyphs;
+    if (subset) {
+        while (!subset->isBitSet(lastIndex - 1) && lastIndex > 0) {
+            --lastIndex;
+        }
+    }
+    AdvanceMetric curRange(0);
+
+    for (int gId = 0; gId <= lastIndex; gId++) {
+        int16_t advance = kInvalidAdvance;
+        if (gId < lastIndex) {
+            if (!subset || 0 == gId || subset->isBitSet(gId)) {
+                advance = (int16_t)cache->getGlyphIDAdvance(gId).fAdvanceX;
+            } else {
+                advance = kDontCareAdvance;
+            }
+        }
+        if (advance == lastAdvance) {
+            repeatedAdvances++;
+            trailingWildCards = 0;
+        } else if (advance == kDontCareAdvance) {
+            wildCardsInRun++;
+            trailingWildCards++;
+        } else if (curRange.fAdvance.count() ==
+                   repeatedAdvances + 1 + wildCardsInRun) {  // All in run.
+            if (lastAdvance == 0) {
+                curRange.fStartId = gId;  // reset
+                curRange.fAdvance.setCount(0);
+                trailingWildCards = 0;
+            } else if (repeatedAdvances + 1 >= 2 || trailingWildCards >= 4) {
+                finish_range(&curRange, gId - 1, AdvanceMetric::kRun);
+                compose_advance_data(curRange, emSize, defaultAdvance, result.get());
+                prevRange = true;
+                curRange = AdvanceMetric(gId);
+                trailingWildCards = 0;
+            }
+            repeatedAdvances = 0;
+            wildCardsInRun = trailingWildCards;
+            trailingWildCards = 0;
+        } else {
+            if (lastAdvance == 0 &&
+                    repeatedAdvances + 1 + wildCardsInRun >= 4) {
+                finish_range(&curRange,
+                            gId - repeatedAdvances - wildCardsInRun - 2,
+                            AdvanceMetric::kRange);
+                compose_advance_data(curRange, emSize, defaultAdvance, result.get());
+                prevRange = true;
+                curRange = AdvanceMetric(gId);
+                trailingWildCards = 0;
+            } else if (trailingWildCards >= 4 && repeatedAdvances + 1 < 2) {
+                finish_range(&curRange, gId - trailingWildCards - 1,
+                            AdvanceMetric::kRange);
+                compose_advance_data(curRange, emSize, defaultAdvance, result.get());
+                prevRange = true;
+                curRange = AdvanceMetric(gId);
+                trailingWildCards = 0;
+            } else if (lastAdvance != 0 &&
+                       (repeatedAdvances + 1 >= 3 ||
+                        (repeatedAdvances + 1 >= 2 && wildCardsInRun >= 3))) {
+                finish_range(&curRange,
+                            gId - repeatedAdvances - wildCardsInRun - 2,
+                            AdvanceMetric::kRange);
+                compose_advance_data(curRange, emSize, defaultAdvance, result.get());
+                curRange =
+                        AdvanceMetric(gId - repeatedAdvances - wildCardsInRun - 1);
+                curRange.fAdvance.append(1, &lastAdvance);
+                finish_range(&curRange, gId - 1, AdvanceMetric::kRun);
+                compose_advance_data(curRange, emSize, defaultAdvance, result.get());
+                prevRange = true;
+                curRange = AdvanceMetric(gId);
+                trailingWildCards = 0;
+            }
+            repeatedAdvances = 0;
+            wildCardsInRun = trailingWildCards;
+            trailingWildCards = 0;
+        }
+        curRange.fAdvance.append(1, &advance);
+        if (advance != kDontCareAdvance) {
+            lastAdvance = advance;
+        }
+    }
+    if (curRange.fStartId == lastIndex) {
+        if (!prevRange) {
+            return nullptr;  // https://crbug.com/567031
+        }
+    } else {
+        finish_range(&curRange, lastIndex - 1, AdvanceMetric::kRange);
+        compose_advance_data(curRange, emSize, defaultAdvance, result.get());
+    }
+    return result;
+}