AK: Add Utf16View for decoding UTF-16 strings

Also includes a way to transcode from and to UTF-8 strings.

3 files changed, 312 insertions, 0 deletions

diff --git a/AK/Forward.h b/AK/Forward.h
index 408b517ad7..8d4a9b5e8a 100644
--- a/AK/Forward.h
+++ b/AK/Forward.h
@@ -29,6 +29,7 @@ class StringView;
 class Time;
 class URL;
 class FlyString;
+class Utf16View;
 class Utf32View;
 class Utf8View;
 class InputStream;
@@ -168,6 +169,7 @@ using AK::StringView;
 using AK::Time;
 using AK::Traits;
 using AK::URL;
+using AK::Utf16View;
 using AK::Utf32View;
 using AK::Utf8View;
 using AK::Vector;
diff --git a/AK/Utf16View.cpp b/AK/Utf16View.cpp
new file mode 100644
index 0000000000..83d95d4543
--- /dev/null
+++ b/AK/Utf16View.cpp
@@ -0,0 +1,197 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/StringBuilder.h>
+#include <AK/StringView.h>
+#include <AK/Utf16View.h>
+#include <AK/Utf8View.h>
+
+namespace AK {
+
+static constexpr u16 high_surrogate_min = 0xd800;
+static constexpr u16 high_surrogate_max = 0xdbff;
+static constexpr u16 low_surrogate_min = 0xdc00;
+static constexpr u16 low_surrogate_max = 0xdfff;
+static constexpr u32 replacement_code_point = 0xfffd;
+static constexpr u32 first_supplementary_plane_code_point = 0x10000;
+
+Vector<u16> utf8_to_utf16(StringView const& utf8_view)
+{
+    return utf8_to_utf16(Utf8View { utf8_view });
+}
+
+Vector<u16> utf8_to_utf16(Utf8View const& utf8_view)
+{
+    Vector<u16> utf16_data;
+
+    for (auto code_point : utf8_view) {
+        if (code_point < first_supplementary_plane_code_point) {
+            utf16_data.append(static_cast<u16>(code_point));
+        } else {
+            code_point -= first_supplementary_plane_code_point;
+            utf16_data.append(static_cast<u16>(high_surrogate_min | (code_point >> 10)));
+            utf16_data.append(static_cast<u16>(low_surrogate_min | (code_point & 0x3ff)));
+        }
+    }
+
+    return utf16_data;
+}
+
+bool Utf16View::is_high_surrogate(u16 code_unit)
+{
+    return (code_unit >= high_surrogate_min) && (code_unit <= high_surrogate_max);
+}
+
+bool Utf16View::is_low_surrogate(u16 code_unit)
+{
+    return (code_unit >= low_surrogate_min) && (code_unit <= low_surrogate_max);
+}
+
+u32 Utf16View::decode_surrogate_pair(u16 high_surrogate, u16 low_surrogate)
+{
+    VERIFY(is_high_surrogate(high_surrogate));
+    VERIFY(is_low_surrogate(low_surrogate));
+
+    return ((high_surrogate - high_surrogate_min) << 10) + (low_surrogate - low_surrogate_min) + first_supplementary_plane_code_point;
+}
+
+String Utf16View::to_utf8(AllowInvalidCodeUnits allow_invalid_code_units) const
+{
+    StringBuilder builder;
+
+    if (allow_invalid_code_units == AllowInvalidCodeUnits::Yes) {
+        for (auto const* ptr = begin_ptr(); ptr < end_ptr(); ++ptr) {
+            if (is_high_surrogate(*ptr)) {
+                auto const* next = ptr + 1;
+
+                if ((next < end_ptr()) && is_low_surrogate(*next)) {
+                    auto code_point = decode_surrogate_pair(*ptr, *next);
+                    builder.append_code_point(code_point);
+                    ++ptr;
+                    continue;
+                }
+            }
+
+            builder.append_code_point(static_cast<u32>(*ptr));
+        }
+    } else {
+        for (auto code_point : *this)
+            builder.append_code_point(code_point);
+    }
+
+    return builder.build();
+}
+
+size_t Utf16View::length_in_code_points() const
+{
+    if (!m_length_in_code_points.has_value())
+        m_length_in_code_points = calculate_length_in_code_points();
+    return *m_length_in_code_points;
+}
+
+u16 Utf16View::code_unit_at(size_t index) const
+{
+    VERIFY(index < length_in_code_units());
+    return m_code_units[index];
+}
+
+Utf16View Utf16View::substring_view(size_t code_unit_offset, size_t code_unit_length) const
+{
+    VERIFY(!Checked<size_t>::addition_would_overflow(code_unit_offset, code_unit_length));
+    VERIFY(code_unit_offset + code_unit_length <= length_in_code_units());
+
+    return Utf16View { m_code_units.slice(code_unit_offset, code_unit_length) };
+}
+
+bool Utf16View::validate(size_t& valid_code_units) const
+{
+    valid_code_units = 0;
+
+    for (auto const* ptr = begin_ptr(); ptr < end_ptr(); ++ptr) {
+        if (is_high_surrogate(*ptr)) {
+            if ((++ptr >= end_ptr()) || !is_low_surrogate(*ptr))
+                return false;
+            ++valid_code_units;
+        } else if (is_low_surrogate(*ptr)) {
+            return false;
+        }
+
+        ++valid_code_units;
+    }
+
+    return true;
+}
+
+size_t Utf16View::calculate_length_in_code_points() const
+{
+    size_t code_points = 0;
+    for ([[maybe_unused]] auto code_point : *this)
+        ++code_points;
+    return code_points;
+}
+
+bool Utf16View::operator==(Utf16View const& other) const
+{
+    if (length_in_code_units() == 0)
+        return other.length_in_code_units() == 0;
+    if (length_in_code_units() != other.length_in_code_units())
+        return false;
+
+    for (size_t i = 0; i < length_in_code_units(); ++i) {
+        if (m_code_units[i] != other.m_code_units[i])
+            return false;
+    }
+
+    return true;
+}
+
+Utf16CodePointIterator& Utf16CodePointIterator::operator++()
+{
+    size_t code_units = length_in_code_units();
+
+    if (code_units > m_remaining_code_units) {
+        // If there aren't enough code units remaining, skip to the end.
+        m_ptr += m_remaining_code_units;
+        m_remaining_code_units = 0;
+    } else {
+        m_ptr += code_units;
+        m_remaining_code_units -= code_units;
+    }
+
+    return *this;
+}
+
+u32 Utf16CodePointIterator::operator*() const
+{
+    VERIFY(m_remaining_code_units > 0);
+
+    if (Utf16View::is_high_surrogate(*m_ptr)) {
+        if ((m_remaining_code_units > 1) && Utf16View::is_low_surrogate(*(m_ptr + 1)))
+            return Utf16View::decode_surrogate_pair(*m_ptr, *(m_ptr + 1));
+        return replacement_code_point;
+    } else if (Utf16View::is_low_surrogate(*m_ptr)) {
+        return replacement_code_point;
+    }
+
+    return static_cast<u32>(*m_ptr);
+}
+
+size_t Utf16CodePointIterator::length_in_code_units() const
+{
+    VERIFY(m_remaining_code_units > 0);
+
+    if (Utf16View::is_high_surrogate(*m_ptr)) {
+        if ((m_remaining_code_units > 1) && Utf16View::is_low_surrogate(*(m_ptr + 1)))
+            return 2;
+    }
+
+    // If this return is reached, either the encoded code point is a valid single code unit, or that
+    // code point is invalid (e.g. began with a low surrogate, or a low surrogate did not follow a
+    // high surrogate). In the latter case, a single replacement code unit will be used.
+    return 1;
+}
+
+}
diff --git a/AK/Utf16View.h b/AK/Utf16View.h
new file mode 100644
index 0000000000..09f269df0f
--- /dev/null
+++ b/AK/Utf16View.h
@@ -0,0 +1,113 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+#include <AK/Optional.h>
+#include <AK/Span.h>
+#include <AK/String.h>
+#include <AK/Types.h>
+#include <AK/Vector.h>
+
+namespace AK {
+
+Vector<u16> utf8_to_utf16(StringView const&);
+Vector<u16> utf8_to_utf16(Utf8View const&);
+
+class Utf16View;
+
+class Utf16CodePointIterator {
+    friend class Utf16View;
+
+public:
+    Utf16CodePointIterator() = default;
+    ~Utf16CodePointIterator() = default;
+
+    bool operator==(Utf16CodePointIterator const& other) const
+    {
+        return (m_ptr == other.m_ptr) && (m_remaining_code_units == other.m_remaining_code_units);
+    }
+
+    bool operator!=(Utf16CodePointIterator const& other) const
+    {
+        return !(*this == other);
+    }
+
+    Utf16CodePointIterator& operator++();
+    u32 operator*() const;
+
+    size_t length_in_code_units() const;
+
+private:
+    Utf16CodePointIterator(u16 const* ptr, size_t length)
+        : m_ptr(ptr)
+        , m_remaining_code_units(length)
+    {
+    }
+
+    u16 const* m_ptr { nullptr };
+    size_t m_remaining_code_units { 0 };
+};
+
+class Utf16View {
+public:
+    static bool is_high_surrogate(u16);
+    static bool is_low_surrogate(u16);
+    static u32 decode_surrogate_pair(u16 high_surrogate, u16 low_surrogate);
+
+    Utf16View() = default;
+    ~Utf16View() = default;
+
+    explicit Utf16View(Span<u16 const> code_units)
+        : m_code_units(code_units)
+    {
+    }
+
+    Utf16View& operator=(Utf16View const&) = default;
+    Utf16View& operator=(Utf16View&&) = default;
+    bool operator==(Utf16View const& other) const;
+
+    enum class AllowInvalidCodeUnits {
+        Yes,
+        No,
+    };
+
+    String to_utf8(AllowInvalidCodeUnits = AllowInvalidCodeUnits::No) const;
+
+    bool is_empty() const { return m_code_units.is_empty(); }
+    size_t length_in_code_units() const { return m_code_units.size(); }
+    size_t length_in_code_points() const;
+
+    Utf16CodePointIterator begin() const { return { begin_ptr(), m_code_units.size() }; }
+    Utf16CodePointIterator end() const { return { end_ptr(), 0 }; }
+
+    u16 const* data() const { return m_code_units.data(); }
+    u16 code_unit_at(size_t index) const;
+
+    Utf16View substring_view(size_t code_unit_offset, size_t code_unit_length) const;
+    Utf16View substring_view(size_t code_unit_offset) const { return substring_view(code_unit_offset, length_in_code_units() - code_unit_offset); }
+
+    bool validate(size_t& valid_code_units) const;
+    bool validate() const
+    {
+        size_t valid_code_units;
+        return validate(valid_code_units);
+    }
+
+private:
+    u16 const* begin_ptr() const { return m_code_units.data(); }
+    u16 const* end_ptr() const { return begin_ptr() + m_code_units.size(); }
+
+    size_t calculate_length_in_code_points() const;
+
+    Span<u16 const> m_code_units;
+    mutable Optional<size_t> m_length_in_code_points;
+};
+
+}
+
+using AK::Utf16View;