Reland: Experimental parsing expression grammar (PEG) template library

experimental/svg/model/SkPEG.h

Index: experimental/svg/model/SkPEG.h
diff --git a/experimental/svg/model/SkPEG.h b/experimental/svg/model/SkPEG.h
new file mode 100644
index 0000000000000000000000000000000000000000..596a5708dc9a76eec97d9c73a59d53a652b4e30e
--- /dev/null
+++ b/experimental/svg/model/SkPEG.h
@@ -0,0 +1,244 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkPEG_DEFINED
+#define SkPEG_DEFINED
+
+#include "SkTArray.h"
+#include "SkTLazy.h"
+
+namespace skpeg {
+
+/**
+ *  The result of an expression match attempt.
+ *
+ *  If the match was successful, |fNext| points to the next unconsumed character in the
+ *  input string, and |fValue| holds an (arbitrarily nested) match result value.
+ *
+ *  Otherwise, |fNext| is nullptr and |fValue| is uninitialized.
+ */
+template <typename V>
+struct MatchResult {
+    MatchResult(std::nullptr_t) : fNext(nullptr) {}
+    MatchResult(const char* next, const V& v) : fNext(next), fValue(&v) {}
+
+    operator bool() const {
+        SkASSERT(fValue.isValid() == SkToBool(fNext));
+        return SkToBool(fNext);
+    }
+
+    const V& operator* () const { return *fValue.get(); }
+    const V* operator->() const { return  fValue.get(); }
+
+    const char* fNext;
+    SkTLazy<V>  fValue;
+};
+
+/**
+ * Optional operator (e?).  Always succeeds.
+ *
+ * If e also matches, then the result of e::Match() is stored in |fValue|.
+ * Otherwise, |fValue| is uninitialized.
+ *
+ */
+template <typename E>
+struct Opt {
+    struct V {
+        V(const typename E::V* v) : fValue(v) {}
+
+        SkTLazy<typename E::V> fValue;
+    };
+    using MatchT = MatchResult<V>;
+
+    static MatchT Match(const char* in) {
+        const auto m = E::Match(in);
+        return m ? MatchT(m.fNext, V(m.fValue.get()))
+                 : MatchT(in, nullptr);
+    }
+};
+
+/**
+ * Helper for selecting the value type of the n-th expression type in the list.
+ */
+template <size_t, typename... Es> struct SelectV;
+
+template <typename E, typename... Es>
+struct SelectV<0, E, Es...> {
+    using V = typename E::V;
+};
+
+template <size_t idx, typename E, typename... Es>
+struct SelectV<idx, E, Es...> {
+    using V = typename SelectV<idx - 1, Es...>::V;
+};
+
+/**
+ * Sequence operator (e0 e1...).
+ *
+ * Succeeds when all expressions match, in sequence.  The subexpression match
+ * results can be accessed via get<INDEX>() -- where get<0> returns the value
+ * of the first expression, and so on.
+ *
+ */
+template <typename... E> struct Seq;
+
+template <>
+struct Seq<> {
+    struct V {};
+    using MatchT = MatchResult<V>;
+
+    static MatchT Match(const char* in) {
+        return MatchT(in, V());
+    }
+};
+
+template <typename E, typename... Es>
+struct Seq<E, Es...> {
+    class V {
+    public:
+        V(const typename E::V& head, const typename Seq<Es...>::V& tail)
+            : fHeadV(head), fTailV(tail) {}
+
+        template <size_t idx, typename std::enable_if<idx == 0, bool>::type = 0>
+        const typename E::V& get() const {
+            return fHeadV;
+        }
+
+        template <size_t idx, typename std::enable_if<idx != 0, bool>::type = 0>
+        const typename SelectV<idx, E, Es...>::V& get() const {
+            return fTailV.template get<idx - 1>();
+        }
+
+    private:
+        typename E::V          fHeadV;
+        typename Seq<Es...>::V fTailV;
+    };
+    using MatchT = MatchResult<V>;
+
+    static MatchT Match(const char* in) {
+        const auto headMatch = E::Match(in);
+        if (!headMatch) {
+            return nullptr;
+        }
+
+        const auto tailMatch = Seq<Es...>::Match(headMatch.fNext);
+        return tailMatch ? MatchT(tailMatch.fNext, V(*headMatch, *tailMatch))
+                         : nullptr;
+    }
+};
+
+/**
+ * Ordered choice operator (e1|e2).
+ *
+ * Succeeds when either e1 or e2 match (e1 is tried first, then e2).
+ *
+ * The (optional) match results are stored in |v1|, |v2|.
+ *
+ */
+template <typename E1, typename E2>
+struct Choice {
+    struct V {
+        V (const typename E1::V* v1, const typename E2::V* v2) : v1(v1), v2(v2)
+        {
+            SkASSERT(!v1 || !v2);
+        }
+
+        SkTLazy<typename E1::V> v1;
+        SkTLazy<typename E2::V> v2;
+    };
+    using MatchT = MatchResult<V>;
+
+    static MatchT Match(const char* in) {
+        if (const auto m1 = E1::Match(in)) {
+            return MatchT(m1.fNext, V(m1.fValue.get(), nullptr));
+        }
+        if (const auto m2 = E2::Match(in)) {
+            return MatchT(m2.fNext, V(nullptr, m2.fValue.get()));
+        }
+        return nullptr;
+    }
+};
+
+/**
+ * Zero-or-more operator (e*).  Always succeeds.
+ *
+ * Matches e greedily, and stores the match results in |fValues|.
+ *
+ */
+template <typename E>
+struct Any {
+    struct V {
+        V(SkTArray<typename E::V>&& vs) : fValues(vs) {}
+
+        SkTArray<typename E::V> fValues;
+    };
+    using MatchT = MatchResult<V>;
+
+    static MatchT Match(const char* in) {
+        SkTArray<typename E::V> values;
+        while (const auto m = E::Match(in)) {
+            in = m.fNext;
+            values.push_back(*m);
+        }
+        return MatchT(in, std::move(values));
+    }
+};
+
+/**
+ * One-or-more operator (e+).
+ *
+ * Same as zero-or-more, except it fails if e doesn't match at least once.
+ *
+ */
+template <typename E>
+using Some = Seq<E, Any<E>>;
+
+/**
+ * End-of-string atom.  Matches \0.
+ */
+struct EOS {
+    struct V {};
+    using MatchT = MatchResult<V>;
+
+    static MatchT Match(const char* in) {
+        return (*in != '\0') ? nullptr : MatchT(in, V());
+    }
+};
+
+
+/**
+ * Literal atom.  Matches a list of char literals.
+ */
+template <char... Cs> struct LIT;
+
+template <>
+struct LIT<> {
+    struct V {};
+    using MatchT = MatchResult<V>;
+
+    static MatchT Match(const char* in) {
+        return MatchT(in, V());
+    }
+};
+
+template <char C, char... Cs>
+struct LIT<C, Cs...> {
+    struct V {};
+    using MatchT = MatchResult<V>;
+
+    static MatchT Match(const char* in) {
+        if (*in != C) {
+            return nullptr;
+        }
+        const auto m = LIT<Cs...>::Match(in + 1);
+        return m ? MatchT(m.fNext, V()) : nullptr;
+    }
+};
+
+} // skpeg ns
+
+#endif // SkPEG_DEFINED