DynamicColor.java

/*
 * Copyright (C) 2022 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.android.material.color.utilities;

import static androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP;
import static java.lang.Math.max;
import static java.lang.Math.min;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.annotation.RestrictTo;
import com.google.errorprone.annotations.Var;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.function.Function;

/**
 * A color that adjusts itself based on UI state, represented by DynamicScheme.
 *
 * <p>This color automatically adjusts to accommodate a desired contrast level, or other adjustments
 * such as differing in light mode versus dark mode, or what the theme is, or what the color that
 * produced the theme is, etc.
 *
 * <p>Colors without backgrounds do not change tone when contrast changes. Colors with backgrounds
 * become closer to their background as contrast lowers, and further when contrast increases.
 *
 * <p>Prefer the static constructors. They provide a much more simple interface, such as requiring
 * just a hexcode, or just a hexcode and a background.
 *
 * <p>Ultimately, each component necessary for calculating a color, adjusting it for a desired
 * contrast level, and ensuring it has a certain lightness/tone difference from another color, is
 * provided by a function that takes a DynamicScheme and returns a value. This ensures ultimate
 * flexibility, any desired behavior of a color for any design system, but it usually unnecessary.
 * See the default constructor for more information.
 *
 * @hide
 */
// Prevent lint for Function.apply not being available on Android before API level 14 (4.0.1).
// "AndroidJdkLibsChecker" for Function, "NewApi" for Function.apply().
// A java_library Bazel rule with an Android constraint cannot skip these warnings without this
// annotation; another solution would be to create an android_library rule and supply
// AndroidManifest with an SDK set higher than 14.
@SuppressWarnings({"AndroidJdkLibsChecker", "NewApi"})
@RestrictTo(LIBRARY_GROUP)
public final class DynamicColor {
  public final String name;
  public final Function<DynamicScheme, TonalPalette> palette;
  public final Function<DynamicScheme, Double> tone;
  public final boolean isBackground;
  public final Function<DynamicScheme, DynamicColor> background;
  public final Function<DynamicScheme, DynamicColor> secondBackground;
  public final ContrastCurve contrastCurve;
  public final Function<DynamicScheme, ToneDeltaPair> toneDeltaPair;

  public final Function<DynamicScheme, Double> opacity;

  private final HashMap<DynamicScheme, Hct> hctCache = new HashMap<>();

  /**
   * A constructor for DynamicColor.
   *
   * <p>_Strongly_ prefer using one of the convenience constructors. This class is arguably too
   * flexible to ensure it can support any scenario. Functional arguments allow overriding without
   * risks that come with subclasses.
   *
   * <p>For example, the default behavior of adjust tone at max contrast to be at a 7.0 ratio with
   * its background is principled and matches accessibility guidance. That does not mean it's the
   * desired approach for _every_ design system, and every color pairing, always, in every case.
   *
   * <p>For opaque colors (colors with alpha = 100%).
   *
   * @param name The name of the dynamic color.
   * @param palette Function that provides a TonalPalette given DynamicScheme. A TonalPalette is
   *     defined by a hue and chroma, so this replaces the need to specify hue/chroma. By providing
   *     a tonal palette, when contrast adjustments are made, intended chroma can be preserved.
   * @param tone Function that provides a tone, given a DynamicScheme.
   * @param isBackground Whether this dynamic color is a background, with some other color as the
   *     foreground.
   * @param background The background of the dynamic color (as a function of a `DynamicScheme`), if
   *     it exists.
   * @param secondBackground A second background of the dynamic color (as a function of a
   *     `DynamicScheme`), if it exists.
   * @param contrastCurve A `ContrastCurve` object specifying how its contrast against its
   *     background should behave in various contrast levels options.
   * @param toneDeltaPair A `ToneDeltaPair` object specifying a tone delta constraint between two
   *     colors. One of them must be the color being constructed.
   */
  public DynamicColor(
      @NonNull String name,
      @NonNull Function<DynamicScheme, TonalPalette> palette,
      @NonNull Function<DynamicScheme, Double> tone,
      boolean isBackground,
      @Nullable Function<DynamicScheme, DynamicColor> background,
      @Nullable Function<DynamicScheme, DynamicColor> secondBackground,
      @Nullable ContrastCurve contrastCurve,
      @Nullable Function<DynamicScheme, ToneDeltaPair> toneDeltaPair) {

    this.name = name;
    this.palette = palette;
    this.tone = tone;
    this.isBackground = isBackground;
    this.background = background;
    this.secondBackground = secondBackground;
    this.contrastCurve = contrastCurve;
    this.toneDeltaPair = toneDeltaPair;
    this.opacity = null;
  }

  public DynamicColor(
      @NonNull String name,
      @NonNull Function<DynamicScheme, TonalPalette> palette,
      @NonNull Function<DynamicScheme, Double> tone,
      boolean isBackground,
      @Nullable Function<DynamicScheme, DynamicColor> background,
      @Nullable Function<DynamicScheme, DynamicColor> secondBackground,
      @Nullable ContrastCurve contrastCurve,
      @Nullable Function<DynamicScheme, ToneDeltaPair> toneDeltaPair,
      @Nullable Function<DynamicScheme, Double> opacity) {
    this.name = name;
    this.palette = palette;
    this.tone = tone;
    this.isBackground = isBackground;
    this.background = background;
    this.secondBackground = secondBackground;
    this.contrastCurve = contrastCurve;
    this.toneDeltaPair = toneDeltaPair;
    this.opacity = opacity;
  }

  /**
   * A convenience constructor for DynamicColor.
   *
   * <p>_Strongly_ prefer using one of the convenience constructors. This class is arguably too
   * flexible to ensure it can support any scenario. Functional arguments allow overriding without
   * risks that come with subclasses.
   *
   * <p>For example, the default behavior of adjust tone at max contrast to be at a 7.0 ratio with
   * its background is principled and matches accessibility guidance. That does not mean it's the
   * desired approach for _every_ design system, and every color pairing, always, in every case.
   *
   * <p>For opaque colors (colors with alpha = 100%).
   *
   * <p>For colors that are not backgrounds, and do not have backgrounds.
   *
   * @param name The name of the dynamic color.
   * @param palette Function that provides a TonalPalette given DynamicScheme. A TonalPalette is
   *     defined by a hue and chroma, so this replaces the need to specify hue/chroma. By providing
   *     a tonal palette, when contrast adjustments are made, intended chroma can be preserved.
   * @param tone Function that provides a tone, given a DynamicScheme.
   */
  @NonNull
  public static DynamicColor fromPalette(
      @NonNull String name,
      @NonNull Function<DynamicScheme, TonalPalette> palette,
      @NonNull Function<DynamicScheme, Double> tone) {
    return new DynamicColor(
        name,
        palette,
        tone,
        /* isBackground= */ false,
        /* background= */ null,
        /* secondBackground= */ null,
        /* contrastCurve= */ null,
        /* toneDeltaPair= */ null);
  }

  /**
   * A convenience constructor for DynamicColor.
   *
   * <p>_Strongly_ prefer using one of the convenience constructors. This class is arguably too
   * flexible to ensure it can support any scenario. Functional arguments allow overriding without
   * risks that come with subclasses.
   *
   * <p>For example, the default behavior of adjust tone at max contrast to be at a 7.0 ratio with
   * its background is principled and matches accessibility guidance. That does not mean it's the
   * desired approach for _every_ design system, and every color pairing, always, in every case.
   *
   * <p>For opaque colors (colors with alpha = 100%).
   *
   * <p>For colors that do not have backgrounds.
   *
   * @param name The name of the dynamic color.
   * @param palette Function that provides a TonalPalette given DynamicScheme. A TonalPalette is
   *     defined by a hue and chroma, so this replaces the need to specify hue/chroma. By providing
   *     a tonal palette, when contrast adjustments are made, intended chroma can be preserved.
   * @param tone Function that provides a tone, given a DynamicScheme.
   * @param isBackground Whether this dynamic color is a background, with some other color as the
   *     foreground.
   */
  @NonNull
  public static DynamicColor fromPalette(
      @NonNull String name,
      @NonNull Function<DynamicScheme, TonalPalette> palette,
      @NonNull Function<DynamicScheme, Double> tone,
      boolean isBackground) {
    return new DynamicColor(
        name,
        palette,
        tone,
        isBackground,
        /* background= */ null,
        /* secondBackground= */ null,
        /* contrastCurve= */ null,
        /* toneDeltaPair= */ null);
  }

  /**
   * Create a DynamicColor from a hex code.
   *
   * <p>Result has no background; thus no support for increasing/decreasing contrast for a11y.
   *
   * @param name The name of the dynamic color.
   * @param argb The source color from which to extract the hue and chroma.
   */
  @NonNull
  public static DynamicColor fromArgb(@NonNull String name, int argb) {
    Hct hct = Hct.fromInt(argb);
    TonalPalette palette = TonalPalette.fromInt(argb);
    return DynamicColor.fromPalette(name, (s) -> palette, (s) -> hct.getTone());
  }

  /**
   * Returns an ARGB integer (i.e. a hex code).
   *
   * @param scheme Defines the conditions of the user interface, for example, whether or not it is
   *     dark mode or light mode, and what the desired contrast level is.
   */
  public int getArgb(@NonNull DynamicScheme scheme) {
    int argb = getHct(scheme).toInt();
    if (opacity == null) {
      return argb;
    }
    double percentage = opacity.apply(scheme);
    int alpha = MathUtils.clampInt(0, 255, (int) Math.round(percentage * 255));
    return (argb & 0x00ffffff) | (alpha << 24);
  }

  /**
   * Returns an HCT object.
   *
   * @param scheme Defines the conditions of the user interface, for example, whether or not it is
   *     dark mode or light mode, and what the desired contrast level is.
   */
  @NonNull
  public Hct getHct(@NonNull DynamicScheme scheme) {
    Hct cachedAnswer = hctCache.get(scheme);
    if (cachedAnswer != null) {
      return cachedAnswer;
    }
    // This is crucial for aesthetics: we aren't simply the taking the standard color
    // and changing its tone for contrast. Rather, we find the tone for contrast, then
    // use the specified chroma from the palette to construct a new color.
    //
    // For example, this enables colors with standard tone of T90, which has limited chroma, to
    // "recover" intended chroma as contrast increases.
    double tone = getTone(scheme);
    Hct answer = palette.apply(scheme).getHct(tone);
    // NOMUTANTS--trivial test with onerous dependency injection requirement.
    if (hctCache.size() > 4) {
      hctCache.clear();
    }
    // NOMUTANTS--trivial test with onerous dependency injection requirement.
    hctCache.put(scheme, answer);
    return answer;
  }

  /** Returns the tone in HCT, ranging from 0 to 100, of the resolved color given scheme. */
  public double getTone(@NonNull DynamicScheme scheme) {
    boolean decreasingContrast = scheme.contrastLevel < 0;

    // Case 1: dual foreground, pair of colors with delta constraint.
    if (toneDeltaPair != null) {
      ToneDeltaPair toneDeltaPair = this.toneDeltaPair.apply(scheme);
      DynamicColor roleA = toneDeltaPair.getRoleA();
      DynamicColor roleB = toneDeltaPair.getRoleB();
      double delta = toneDeltaPair.getDelta();
      TonePolarity polarity = toneDeltaPair.getPolarity();
      boolean stayTogether = toneDeltaPair.getStayTogether();

      DynamicColor bg = background.apply(scheme);
      double bgTone = bg.getTone(scheme);

      boolean aIsNearer =
          (polarity == TonePolarity.NEARER
              || (polarity == TonePolarity.LIGHTER && !scheme.isDark)
              || (polarity == TonePolarity.DARKER && scheme.isDark));
      DynamicColor nearer = aIsNearer ? roleA : roleB;
      DynamicColor farther = aIsNearer ? roleB : roleA;
      boolean amNearer = name.equals(nearer.name);
      double expansionDir = scheme.isDark ? 1 : -1;

      // 1st round: solve to min, each
      double nContrast = nearer.contrastCurve.getContrast(scheme.contrastLevel);
      double fContrast = farther.contrastCurve.getContrast(scheme.contrastLevel);

      // If a color is good enough, it is not adjusted.
      // Initial and adjusted tones for `nearer`
      double nInitialTone = nearer.tone.apply(scheme);
      @Var
      double nTone =
          Contrast.ratioOfTones(bgTone, nInitialTone) >= nContrast
              ? nInitialTone
              : DynamicColor.foregroundTone(bgTone, nContrast);
      // Initial and adjusted tones for `farther`
      double fInitialTone = farther.tone.apply(scheme);
      @Var
      double fTone =
          Contrast.ratioOfTones(bgTone, fInitialTone) >= fContrast
              ? fInitialTone
              : DynamicColor.foregroundTone(bgTone, fContrast);

      if (decreasingContrast) {
        // If decreasing contrast, adjust color to the "bare minimum"
        // that satisfies contrast.
        nTone = DynamicColor.foregroundTone(bgTone, nContrast);
        fTone = DynamicColor.foregroundTone(bgTone, fContrast);
      }

      // If constraint is not satisfied, try another round.
      if ((fTone - nTone) * expansionDir < delta) {
        // 2nd round: expand farther to match delta.
        fTone = MathUtils.clampDouble(0, 100, nTone + delta * expansionDir);
        // If constraint is not satisfied, try another round.
        if ((fTone - nTone) * expansionDir < delta) {
          // 3rd round: contract nearer to match delta.
          nTone = MathUtils.clampDouble(0, 100, fTone - delta * expansionDir);
        }
      }

      // Avoids the 50-59 awkward zone.
      if (50 <= nTone && nTone < 60) {
        // If `nearer` is in the awkward zone, move it away, together with
        // `farther`.
        if (expansionDir > 0) {
          nTone = 60;
          fTone = max(fTone, nTone + delta * expansionDir);
        } else {
          nTone = 49;
          fTone = min(fTone, nTone + delta * expansionDir);
        }
      } else if (50 <= fTone && fTone < 60) {
        if (stayTogether) {
          // Fixes both, to avoid two colors on opposite sides of the "awkward
          // zone".
          if (expansionDir > 0) {
            nTone = 60;
            fTone = max(fTone, nTone + delta * expansionDir);
          } else {
            nTone = 49;
            fTone = min(fTone, nTone + delta * expansionDir);
          }
        } else {
          // Not required to stay together; fixes just one.
          if (expansionDir > 0) {
            fTone = 60;
          } else {
            fTone = 49;
          }
        }
      }

      // Returns `nTone` if this color is `nearer`, otherwise `fTone`.
      return amNearer ? nTone : fTone;
    } else {
      // Case 2: No contrast pair; just solve for itself.
      @Var double answer = tone.apply(scheme);

      if (background == null) {
        return answer; // No adjustment for colors with no background.
      }

      double bgTone = background.apply(scheme).getTone(scheme);

      double desiredRatio = contrastCurve.getContrast(scheme.contrastLevel);

      if (Contrast.ratioOfTones(bgTone, answer) >= desiredRatio) {
        // Don't "improve" what's good enough.
      } else {
        // Rough improvement.
        answer = DynamicColor.foregroundTone(bgTone, desiredRatio);
      }

      if (decreasingContrast) {
        answer = DynamicColor.foregroundTone(bgTone, desiredRatio);
      }

      if (isBackground && 50 <= answer && answer < 60) {
        // Must adjust
        if (Contrast.ratioOfTones(49, bgTone) >= desiredRatio) {
          answer = 49;
        } else {
          answer = 60;
        }
      }

      if (secondBackground != null) {
        // Case 3: Adjust for dual backgrounds.

        double bgTone1 = background.apply(scheme).getTone(scheme);
        double bgTone2 = secondBackground.apply(scheme).getTone(scheme);

        double upper = max(bgTone1, bgTone2);
        double lower = min(bgTone1, bgTone2);

        if (Contrast.ratioOfTones(upper, answer) >= desiredRatio
            && Contrast.ratioOfTones(lower, answer) >= desiredRatio) {
          return answer;
        }

        // The darkest light tone that satisfies the desired ratio,
        // or -1 if such ratio cannot be reached.
        double lightOption = Contrast.lighter(upper, desiredRatio);

        // The lightest dark tone that satisfies the desired ratio,
        // or -1 if such ratio cannot be reached.
        double darkOption = Contrast.darker(lower, desiredRatio);

        // Tones suitable for the foreground.
        ArrayList<Double> availables = new ArrayList<>();
        if (lightOption != -1) {
          availables.add(lightOption);
        }
        if (darkOption != -1) {
          availables.add(darkOption);
        }

        boolean prefersLight =
            DynamicColor.tonePrefersLightForeground(bgTone1)
                || DynamicColor.tonePrefersLightForeground(bgTone2);
        if (prefersLight) {
          return (lightOption == -1) ? 100 : lightOption;
        }
        if (availables.size() == 1) {
          return availables.get(0);
        }
        return (darkOption == -1) ? 0 : darkOption;
      }

      return answer;
    }
  }

  /**
   * Given a background tone, find a foreground tone, while ensuring they reach a contrast ratio
   * that is as close to ratio as possible.
   */
  public static double foregroundTone(double bgTone, double ratio) {
    double lighterTone = Contrast.lighterUnsafe(bgTone, ratio);
    double darkerTone = Contrast.darkerUnsafe(bgTone, ratio);
    double lighterRatio = Contrast.ratioOfTones(lighterTone, bgTone);
    double darkerRatio = Contrast.ratioOfTones(darkerTone, bgTone);
    boolean preferLighter = tonePrefersLightForeground(bgTone);

    if (preferLighter) {
      // "Neglible difference" handles an edge case where the initial contrast ratio is high
      // (ex. 13.0), and the ratio passed to the function is that high ratio, and both the lighter
      // and darker ratio fails to pass that ratio.
      //
      // This was observed with Tonal Spot's On Primary Container turning black momentarily between
      // high and max contrast in light mode. PC's standard tone was T90, OPC's was T10, it was
      // light mode, and the contrast level was 0.6568521221032331.
      boolean negligibleDifference =
          Math.abs(lighterRatio - darkerRatio) < 0.1 && lighterRatio < ratio && darkerRatio < ratio;
      if (lighterRatio >= ratio || lighterRatio >= darkerRatio || negligibleDifference) {
        return lighterTone;
      } else {
        return darkerTone;
      }
    } else {
      return darkerRatio >= ratio || darkerRatio >= lighterRatio ? darkerTone : lighterTone;
    }
  }

  /**
   * Adjust a tone down such that white has 4.5 contrast, if the tone is reasonably close to
   * supporting it.
   */
  public static double enableLightForeground(double tone) {
    if (tonePrefersLightForeground(tone) && !toneAllowsLightForeground(tone)) {
      return 49.0;
    }
    return tone;
  }

  /**
   * People prefer white foregrounds on ~T60-70. Observed over time, and also by Andrew Somers
   * during research for APCA.
   *
   * <p>T60 used as to create the smallest discontinuity possible when skipping down to T49 in order
   * to ensure light foregrounds.
   *
   * <p>Since `tertiaryContainer` in dark monochrome scheme requires a tone of 60, it should not be
   * adjusted. Therefore, 60 is excluded here.
   */
  public static boolean tonePrefersLightForeground(double tone) {
    return Math.round(tone) < 60;
  }

  /** Tones less than ~T50 always permit white at 4.5 contrast. */
  public static boolean toneAllowsLightForeground(double tone) {
    return Math.round(tone) <= 49;
  }
}