// Copyright (c) Six Labors.
// Licensed under the Six Labors Split License.

using System;
using System.Diagnostics.CodeAnalysis;
using System.Numerics;
using SixLabors.Fonts.Tables.AdvancedTypographic;
using SixLabors.Fonts.Tables.AdvancedTypographic.Variations;
using SixLabors.Fonts.Unicode;

namespace SixLabors.Fonts {
  /// <summary>
  /// Represents a font face with metrics, which is a set of glyphs with a specific style (regular, italic, bold etc).
  /// </summary>
  public abstract class FontMetrics
  {
      internal FontMetrics()
      {
      }

      /// <summary>
      /// Gets the basic description of the face.
      /// </summary>
      public abstract FontDescription Description { get; }

      /// <summary>
      /// Gets the number of font units per EM square for this face.
      /// </summary>
      public abstract ushort UnitsPerEm { get; }

      /// <summary>
      /// Gets the scale factor that is applied to all glyphs in this face.
      /// Calculated as 72 * <see cref="UnitsPerEm"/> so that 1pt = 1px.
      /// </summary>
      public abstract float ScaleFactor { get; }

      /// <summary>
      /// Gets the metrics specific to horizontal text.
      /// </summary>
      public abstract HorizontalMetrics HorizontalMetrics { get; }

      /// <summary>
      /// Gets the metrics specific to vertical text.
      /// </summary>
      public abstract VerticalMetrics VerticalMetrics { get; }

      /// <summary>
      /// Gets the recommended horizontal size in font design units for subscripts for this font.
      /// </summary>
      public abstract short SubscriptXSize { get; }

      /// <summary>
      /// Gets the recommended vertical size in font design units for subscripts for this font.
      /// </summary>
      public abstract short SubscriptYSize { get; }

      /// <summary>
      /// Gets the recommended horizontal offset in font design units for subscripts for this font.
      /// </summary>
      public abstract short SubscriptXOffset { get; }

      /// <summary>
      /// Gets the recommended vertical offset in font design units for subscripts for this font.
      /// </summary>
      public abstract short SubscriptYOffset { get; }

      /// <summary>
      /// Gets the recommended horizontal size in font design units for superscripts for this font.
      /// </summary>
      public abstract short SuperscriptXSize { get; }

      /// <summary>
      /// Gets the recommended vertical size in font design units for superscripts for this font.
      /// </summary>
      public abstract short SuperscriptYSize { get; }

      /// <summary>
      /// Gets the recommended horizontal offset in font design units for superscripts for this font.
      /// </summary>
      public abstract short SuperscriptXOffset { get; }

      /// <summary>
      /// Gets the recommended vertical offset in font design units for superscripts for this font.
      /// </summary>
      public abstract short SuperscriptYOffset { get; }

      /// <summary>
      /// Gets thickness of the strikeout stroke in font design units.
      /// </summary>
      public abstract short StrikeoutSize { get; }

      /// <summary>
      /// Gets the position of the top of the strikeout stroke relative to the baseline in font design units.
      /// </summary>
      public abstract short StrikeoutPosition { get; }

      /// <summary>
      /// Gets the suggested distance of the top of the underline from the baseline (negative values indicate below baseline).
      /// </summary>
      public abstract short UnderlinePosition { get; }

      /// <summary>
      /// Gets the suggested values for the underline thickness. In general, the underline thickness should match the thickness of
      /// the underscore character (U+005F LOW LINE), and should also match the strikeout thickness, which is specified in the OS/2 table.
      /// </summary>
      public abstract short UnderlineThickness { get; }

      /// <summary>
      /// Gets the italic angle in counter-clockwise degrees from the vertical. Zero for upright text, negative for text that leans to the right (forward).
      /// </summary>
      public abstract float ItalicAngle { get; }

      /// <summary>
      /// Gets the specified glyph id matching the codepoint.
      /// </summary>
      /// <param name="codePoint">The codepoint.</param>
      /// <param name="glyphId">
      /// When this method returns, contains the glyph id associated with the specified codepoint,
      /// if the codepoint is found; otherwise, <value>0</value>.
      /// This parameter is passed uninitialized.
      /// </param>
      /// <returns>
      /// <see langword="true"/> if the face contains a glyph for the specified codepoint; otherwise, <see langword="false"/>.
      /// </returns>
      internal abstract bool TryGetGlyphId(CodePoint codePoint, out ushort glyphId);

      /// <summary>
      /// Gets the specified glyph id matching the codepoint pair.
      /// </summary>
      /// <param name="codePoint">The codepoint.</param>
      /// <param name="nextCodePoint">The next codepoint. Can be null.</param>
      /// <param name="glyphId">
      /// When this method returns, contains the glyph id associated with the specified codepoint,
      /// if the codepoint is found; otherwise, <value>0</value>.
      /// This parameter is passed uninitialized.
      /// </param>
      /// <param name="skipNextCodePoint">
      /// When this method return, contains a value indicating whether the next codepoint should be skipped.
      /// </param>
      /// <returns>
      /// <see langword="true"/> if the face contains a glyph for the specified codepoint; otherwise, <see langword="false"/>.
      /// </returns>
      internal abstract bool TryGetGlyphId(CodePoint codePoint, CodePoint? nextCodePoint, out ushort glyphId, out bool skipNextCodePoint);

      /// <summary>
      /// Gets the specified glyph id matching the codepoint.
      /// </summary>
      /// <param name="glyphId">The glyph identifier.</param>
      /// <param name="codePoint">
      /// When this method returns, contains the codepoint associated with the specified glyph id,
      /// if the glyph id is found; otherwise, <value>default</value>.
      /// </param>
      /// <returns>
      /// <see langword="true"/> if the face contains a codepoint for the specified glyph id; otherwise, <see langword="false"/>.
      /// </returns>
      internal abstract bool TryGetCodePoint(ushort glyphId, out CodePoint codePoint);

      /// <summary>
      /// Tries to get the glyph class for a given glyph id.
      /// The font needs to have a GDEF table defined.
      /// </summary>
      /// <param name="glyphId">The glyph identifier.</param>
      /// <param name="glyphClass">The glyph class.</param>
      /// <returns>true, if the glyph class could be retrieved.</returns>
      internal abstract bool TryGetGlyphClass(ushort glyphId, [NotNullWhen(true)] out GlyphClassDef? glyphClass);

      /// <summary>
      /// Tries to get the mark attachment class for a given glyph id.
      /// The font needs to have a GDEF table defined.
      /// </summary>
      /// <param name="glyphId">The glyph identifier.</param>
      /// <param name="markAttachmentClass">The mark attachment class.</param>
      /// <returns>true, if the mark attachment class could be retrieved.</returns>
      internal abstract bool TryGetMarkAttachmentClass(ushort glyphId, [NotNullWhen(true)] out GlyphClassDef? markAttachmentClass);

      /// <summary>
      /// Tries to get the variation axes that this font supports.
      /// The font needs to have a fvar table.
      /// </summary>
      /// <param name="variationAxes">A read-only memory region containing the variation axes.</param>
      /// <returns>True, if fvar table is present.</returns>
      public abstract bool TryGetVariationAxes(out ReadOnlyMemory<VariationAxis> variationAxes);

      /// <summary>
      /// Returns a value indicating whether the specified glyph is in the given mark filtering set.
      /// The font needs to have a GDEF table defined.
      /// </summary>
      /// <param name="markGlyphSetIndex">The mark glyph set index.</param>
      /// <param name="glyphId">The glyph identifier.</param>
      /// <returns>
      /// true, if the glyph is in the mark filtering set.
      /// </returns>
      internal abstract bool IsInMarkFilteringSet(ushort markGlyphSetIndex, ushort glyphId);

      /// <summary>
      /// Gets the glyph metrics for a given code point.
      /// </summary>
      /// <param name="codePoint">The Unicode code point to get the glyph for.</param>
      /// <param name="textAttributes">The text attributes applied to the glyph.</param>
      /// <param name="textDecorations">The text decorations applied to the glyph.</param>
      /// <param name="layoutMode">The layout mode applied to the glyph.</param>
      /// <param name="support">Options for enabling color font support during layout and rendering.</param>
      /// <param name="metrics">
      /// When this method returns, contains the metrics for the given codepoint and color support if the metrics
      /// are found; otherwise the default value. This parameter is passed uninitialized.
      /// </param>
      /// <returns>
      /// <see langword="true"/> if the face contains glyph metrics for the specified codepoint; otherwise, <see langword="false"/>.
      /// </returns>
      public abstract bool TryGetGlyphMetrics(
          CodePoint codePoint,
          TextAttributes textAttributes,
          TextDecorations textDecorations,
          LayoutMode layoutMode,
          ColorFontSupport support,
          [NotNullWhen(true)] out FontGlyphMetrics? metrics);

      /// <summary>
      /// Gets the unicode codepoints for which a glyph exists in the font.
      /// </summary>
      /// <returns>A read-only memory region containing the available codepoints.</returns>
      public abstract ReadOnlyMemory<CodePoint> GetAvailableCodePoints();

      /// <summary>
      /// Gets the glyph metrics for a given code point and glyph id.
      /// </summary>
      /// <param name="codePoint">The Unicode codepoint.</param>
      /// <param name="glyphId">
      /// The previously matched or substituted glyph id for the codepoint in the face.
      /// If this value equals <value>0</value> the default fallback metrics are returned.
      /// </param>
      /// <param name="textAttributes">The text attributes applied to the glyph.</param>
      /// <param name="textDecorations">The text decorations applied to the glyph.</param>
      /// <param name="layoutMode">The layout mode applied to the glyph.</param>
      /// <param name="colorSupport">Options for enabling color font support during layout and rendering.</param>
      /// <returns>The font glyph metrics.</returns>
      internal abstract FontGlyphMetrics GetGlyphMetrics(
          CodePoint codePoint,
          ushort glyphId,
          TextAttributes textAttributes,
          TextDecorations textDecorations,
          LayoutMode layoutMode,
          ColorFontSupport colorSupport);

      /// <summary>
      /// Tries to get the GSUB table.
      /// </summary>
      /// <param name="gSubTable">The GSUB table.</param>
      /// <returns>true, if the glyph class could be retrieved.</returns>
      internal abstract bool TryGetGSubTable([NotNullWhen(true)] out GSubTable? gSubTable);

      /// <summary>
      /// Applies any available substitutions to the collection of glyphs.
      /// </summary>
      /// <param name="collection">The glyph substitution collection.</param>
      internal abstract void ApplySubstitution(GlyphSubstitutionCollection collection);

      /// <summary>
      /// Gets the amount, in font units, the <paramref name="currentId"/> glyph should be offset if it is followed by
      /// the <paramref name="nextId"/> glyph.
      /// </summary>
      /// <param name="currentId">The current glyph id.</param>
      /// <param name="nextId">The next glyph id.</param>
      /// <param name="vector">
      /// When this method returns, contains the offset, in font units, that should be applied to the
      /// <paramref name="currentId"/> glyph, if the offset is found; otherwise the default vector value.
      /// This parameter is passed uninitialized.
      /// </param>
      /// <returns>
      /// <see langword="true"/> if the face contains and offset for the glyph combination; otherwise, <see langword="false"/>.
      /// </returns>
      internal abstract bool TryGetKerningOffset(ushort currentId, ushort nextId, out Vector2 vector);

      /// <summary>
      /// Applies any available positioning updates to the collection of glyphs.
      /// </summary>
      /// <param name="collection">The glyph positioning collection.</param>
      internal abstract void UpdatePositions(GlyphPositioningCollection collection);

      /// <summary>
      /// Computes a GPOS/GSUB variation delta for the given packed VariationIndex.
      /// The delta is computed using the GDEF ItemVariationStore and the current
      /// variation coordinates from the GlyphVariationProcessor.
      /// </summary>
      /// <param name="packedVariationIndex">
      /// The packed VariationIndex: (outerIndex &lt;&lt; 16) | innerIndex.
      /// A value of 0 returns 0.
      /// </param>
      /// <returns>The delta value in design units, or 0 if no variation data is available.</returns>
      internal abstract float GetGDefVariationDelta(uint packedVariationIndex);

      /// <summary>
      /// Gets the normalized variation coordinates for this font instance.
      /// Returns an empty span for non-variable fonts or fonts at default coordinates.
      /// </summary>
      /// <returns>The normalized coordinates, or an empty span.</returns>
      internal abstract ReadOnlySpan<float> GetNormalizedCoordinates();
  }
}