import { CoupleRole, DancerIdentity, Rotation, normalizeRotation } from "./danceCommon.js";
import { DancerSetPosition, DancersSetPositions, Hand, OffsetDistance, OffsetEquals, OffsetMinus, OffsetPlus, OffsetRotate, OffsetTimes, OffsetTranspose, offsetZero, setHeight } from "./rendererConstants.js";
import { Offset, leftShoulder, rightShoulder, degreesToRadians, radiansToDegrees } from "./rendererConstants.js";

export enum AnimationKind {
  Linear = "Linear",
  Rotation = "Rotation",
}

export interface RotationAround {
  center: Offset,
  width: number,
  height: number,
};

export abstract class AnimationSegment {
  public readonly beats: number;
  // Start position is redundant... but allows AnimationSegments to be defined indepent of their surroundings.
  public readonly startPosition: DancerSetPosition;
  public readonly endPosition: DancerSetPosition;

  protected constructor(beats: number, startPosition: DancerSetPosition, endPosition: DancerSetPosition) {
    this.beats = beats;
    this.startPosition = startPosition;
    this.endPosition = endPosition;
  }

  abstract interpolateOffset(progress: number): Offset;

  // TODO Not sure this interpolation is actually right for arms...
  interpolateHandOffset(progress: number, hand: Hand) : Offset | undefined {
    let prev = hand === Hand.Left ? this.startPosition.leftArmEnd : this.startPosition.rightArmEnd;
    let next = hand === Hand.Left ? this.endPosition.leftArmEnd : this.endPosition.rightArmEnd;

    if (!prev && !next) return undefined;
    prev ??= hand.shoulderPosition();
    next ??= hand.shoulderPosition();

    return interpolateLinearOffset(progress, prev, next);
  }

  interpolateRotation(progress: number) {
    return interpolateLinear(progress, this.startPosition.rotation, this.endPosition.rotation);
  }

  abstract drawDebug(ctx: CanvasRenderingContext2D, progress: number);

  positionAtFraction(progress: number) : DancerSetPosition {
    return {
      position: this.interpolateOffset(progress),
      rotation: this.interpolateRotation(progress),
      leftArmEnd: this.interpolateHandOffset(progress, Hand.Left),
      rightArmEnd: this.interpolateHandOffset(progress, Hand.Right),
      drawDebug: (ctx) => this.drawDebug(ctx, progress),
    }
  }
}

function interpolateLinear(progress: number, prev: number, next: number): number {
  return next * progress + prev * (1 - progress);
}

function interpolateLinearOffset(progress: number, prev: Offset, next: Offset): Offset {
  return {
    x: interpolateLinear(progress, prev.x, next.x),
    y: interpolateLinear(progress, prev.y, next.y)
  };
}

export class LinearAnimationSegment extends AnimationSegment {
  constructor({ beats, startPosition, endPosition }: {
    beats: number;
    startPosition: DancerSetPosition;
    endPosition: DancerSetPosition;
  }) {
    super(beats, startPosition, endPosition);
  }

  public static standStill(startAndEndPosition: DancerSetPosition, beats?: number) {
    return new LinearAnimationSegment({
      beats: beats ?? 0,
      startPosition: startAndEndPosition,
      endPosition: startAndEndPosition
    });
  }

  override interpolateOffset(progress: number): Offset {
    return interpolateLinearOffset(progress, this.startPosition.position, this.endPosition.position);
  }

  override drawDebug(ctx: CanvasRenderingContext2D, progress: number) {
    ctx.beginPath();
    ctx.moveTo(this.startPosition.position.x, this.startPosition.position.y);
    ctx.lineTo(this.endPosition.position.x, this.endPosition.position.y);
    ctx.stroke();
  }
}

export interface AnimationTransitionFlags {
  rotation?: boolean;
  hands?: boolean;
  handsDuring?: "Actual" | "None" | "Start" | "End" | Map<Hand, Offset>;
}
export class TransitionAnimationSegment extends AnimationSegment {
  private readonly actualAnimation: AnimationSegment;
  private readonly flags: AnimationTransitionFlags;
  private readonly startTransitionProgress: number;
  private readonly endTransitionProgress: number;
  private readonly startRotation: number;
  private readonly endRotation: number;

  constructor({ actualAnimation, flags, startTransitionBeats, endTransitionBeats }: {
    actualAnimation: AnimationSegment;
    flags: AnimationTransitionFlags;
    startTransitionBeats: number;
    endTransitionBeats?: number;
  }) {
    super(actualAnimation.beats, actualAnimation.startPosition, actualAnimation.endPosition);

    this.actualAnimation = actualAnimation;
    this.flags = flags;
    this.startTransitionProgress = startTransitionBeats / actualAnimation.beats;
    this.endTransitionProgress = endTransitionBeats === undefined ? this.startTransitionProgress : endTransitionBeats / actualAnimation.beats;

    this.startRotation = this.startPosition.rotation;
    this.endRotation = this.endPosition.rotation;
    if (this.flags.rotation) {
      const actualStart = this.actualAnimation.interpolateRotation(0);
      const actualEnd = this.actualAnimation.interpolateRotation(1);

      const transitionStart = this.actualAnimation.interpolateRotation(this.startTransitionProgress);
      const transitionEnd = this.actualAnimation.interpolateRotation(1 - this.endTransitionProgress);

      if (actualEnd > actualStart) {
        while (transitionStart <= this.startRotation - 180) {
          this.startRotation -= 360;
        }
        while (transitionEnd >= this.endRotation + 180) {
          this.endRotation += 360;
        }
      } else if (actualEnd < actualStart) {
        while (transitionStart >= this.startRotation + 180) {
          this.startRotation += 360;
        }
        while (transitionEnd <= this.endRotation - 180) {
          this.endRotation -= 360;
        }
      }

      // Transitions should be short adjustments, not spins.
      while (transitionStart - this.startRotation < -180) {
        this.startRotation -= 360;
      }
      while (transitionStart - this.startRotation > 180) {
        this.startRotation += 360;
      }
      while (transitionEnd - this.endRotation < -180) {
        this.endRotation -= 360;
      }
      while (transitionEnd - this.endRotation > 180) {
        this.endRotation += 360;
      }
    }
  }

  override interpolateOffset(progress: number): Offset {
    return this.actualAnimation.interpolateOffset(progress);
  }

  override interpolateRotation(progress: number): number {
    const actualRotation = this.actualAnimation.interpolateRotation(progress);

    if (this.flags.rotation) {
      if (progress < this.startTransitionProgress) {
        return interpolateLinear(progress / this.startTransitionProgress, this.startRotation, actualRotation);
      } else if ((1 - progress) < this.endTransitionProgress) {
        return interpolateLinear((1 - progress) / this.endTransitionProgress, this.endRotation, actualRotation);
      }
    }

    return actualRotation;
  }

  override interpolateHandOffset(progress: number, hand: Hand): Offset | undefined {
    const startHand = hand === Hand.Left
      ? this.startPosition.leftArmEnd ?? leftShoulder
      : this.startPosition.rightArmEnd ?? rightShoulder;
    const endHand = hand === Hand.Left
      ? this.endPosition.leftArmEnd ?? leftShoulder
      : this.endPosition.rightArmEnd ?? rightShoulder;
    const actualHandOffset = this.flags.handsDuring === undefined || this.flags.handsDuring === "Actual"
      ? this.actualAnimation.interpolateHandOffset(progress, hand)
      : this.flags.handsDuring === "None"
      ? hand.shoulderPosition()
      : this.flags.handsDuring === "Start"
      ? startHand
      : this.flags.handsDuring === "End"
      ? endHand
      : this.flags.handsDuring instanceof Map
      ? this.flags.handsDuring.get(hand)
      : (() => { throw "Unexpected handsDuring: " + this.flags.handsDuring; })();

    if (this.flags.hands) {
      if (progress < this.startTransitionProgress) {
        const startHand = hand === Hand.Left
          ? this.startPosition.leftArmEnd ?? leftShoulder
          : this.startPosition.rightArmEnd ?? rightShoulder;
        return interpolateLinearOffset(progress / this.startTransitionProgress, startHand, actualHandOffset ?? hand.shoulderPosition());
      } else if (1 - progress < this.endTransitionProgress) {
        const endHand = hand === Hand.Left
          ? this.endPosition.leftArmEnd ?? leftShoulder
          : this.endPosition.rightArmEnd ?? rightShoulder;
        return interpolateLinearOffset((1 - progress) / this.endTransitionProgress, endHand, actualHandOffset ?? hand.shoulderPosition());
      }
    }

    return actualHandOffset;
  }

  override drawDebug(ctx: CanvasRenderingContext2D, progress: number) {
    // TODO display transition somehow?
    this.actualAnimation.drawDebug(ctx, progress);
  }
}

// TODO Not sure this really belongs here instead of on some Semantic* type.
export enum RotationAnimationFacing {
  Linear = "Linear", // Default, linearly interpolate.
  Center = "Center", // Always face the center.
  CenterRelative = "CenterRelative", // Always face the center.
  Forward = "Forward", // Always face the direction of the rotation.
  Backward = "Backward", // Opposite of forward.
  Start = "Start", // Stay facing the same direction as at the beginning.
}
export interface CloserDuringRotation {
  minDistance: number,
  transitionBeats: number,
}
interface Closer {
  middleDistance: number,
  transitionProgress: number,
}
export interface HandAnimation {
  kind: "Linear" | "Center" | "CenterUntilPassed" | "None" | "Start" | "End"
}

export class RotationAnimationSegment extends AnimationSegment {
  private readonly xRadius: number;
  private readonly yRadius: number;
  private readonly startDistance: number;
  private readonly endDistance: number;
  private readonly startRotation: number;
  private readonly endRotation: number;
  private readonly center: Offset;
  private readonly facing: RotationAnimationFacing;
  private readonly startFacing: Rotation;
  private readonly closer?: Closer;
  private readonly hands: Map<Hand, HandAnimation>;
  private readonly centerRelativeTo: number;

  constructor({ beats, startPosition, endPosition, rotation, around, facing, closer, hands, centerRelativeTo }: {
    beats: number;
    startPosition: DancerSetPosition;
    endPosition: DancerSetPosition;
    rotation: number;
    around: RotationAround;
    facing: RotationAnimationFacing;
    closer?: CloserDuringRotation;
    hands?: Map<Hand, HandAnimation>;
    centerRelativeTo?: number;
  }) {
    super(beats, startPosition, endPosition);

    this.hands = new Map<Hand, HandAnimation>(
      [Hand.Left, Hand.Right].map(h => [h, hands?.get(h) ?? { kind: "Linear" }]));

    this.facing = facing;
    this.xRadius = around.width / 2;
    this.yRadius = around.height / 2;
    this.center = around.center;
    let positionToDegrees = (pos: Offset): number => {
      const radians = Math.atan2(
        (pos.y - around.center.y) / this.yRadius,
        (pos.x - around.center.x) / this.xRadius
      );
      // Canvas rotation is backwards of what we expect, so take the negative here.
      return -radiansToDegrees(radians);
    }
    this.startRotation = positionToDegrees(startPosition.position);
    this.startFacing = startPosition.rotation;
    this.centerRelativeTo = centerRelativeTo ?? this.startFacing;
    const actualRotation = normalizeRotation(positionToDegrees(endPosition.position) - this.startRotation,
      rotation);
    this.endRotation = this.startRotation + actualRotation;

    let positionToNormalizedDistance = (pos: Offset): number => {
      return Math.sqrt(Math.pow((pos.y - around.center.y) / this.yRadius, 2)
                     + Math.pow((pos.x - around.center.x) / this.xRadius, 2));
    }
    this.startDistance = positionToNormalizedDistance(startPosition.position);
    this.endDistance = positionToNormalizedDistance(endPosition.position);
    if (closer && (this.startDistance > closer.minDistance || this.endDistance > closer.minDistance)) {
      this.closer = {
        middleDistance: closer.minDistance,
        transitionProgress: closer.transitionBeats / beats,
      };
    }
  }

  override interpolateOffset(progress: number): Offset {
    const radians = -degreesToRadians(interpolateLinear(progress, this.startRotation, this.endRotation));
    let distance: number;
    if (!this.closer) {
      distance = interpolateLinear(progress, this.startDistance, this.endDistance);
    } else {
      if (progress <= this.closer.transitionProgress) {
        distance = interpolateLinear(progress / this.closer.transitionProgress, this.startDistance, this.closer.middleDistance);
      } else if ((1 - progress) <= this.closer.transitionProgress) {
        distance = interpolateLinear((1 - progress) / this.closer.transitionProgress, this.endDistance, this.closer.middleDistance);
      } else {
        distance = this.closer.middleDistance;
      }
    }
    return {
      x: this.center.x + this.xRadius * Math.cos(radians) * distance,
      y: this.center.y + this.yRadius * Math.sin(radians) * distance,
    }
  }

  override interpolateRotation(progress: number): number {
    if (this.facing === RotationAnimationFacing.Linear) {
      return super.interpolateRotation(progress);
    } else {
      const degrees = interpolateLinear(progress, this.startRotation, this.endRotation);
      switch (this.facing) {
        case RotationAnimationFacing.Start:
          return this.startFacing;
        case RotationAnimationFacing.Center:
          return degrees - 90;
        case RotationAnimationFacing.CenterRelative:
          return degrees - this.startRotation + this.centerRelativeTo;
        case RotationAnimationFacing.Forward:
        case RotationAnimationFacing.Backward:
          return (this.endRotation > this.startRotation === (this.facing === RotationAnimationFacing.Forward))
            ? degrees + 180
            : degrees;
      }
    }
  }

  override interpolateHandOffset(progress: number, hand: Hand): Offset | undefined {
    const handAnim : HandAnimation = this.hands.get(hand)!;
    switch (handAnim.kind) {
      case "Start":
        return hand === Hand.Left ? this.startPosition.leftArmEnd : this.startPosition.rightArmEnd;
      case "End":
        return hand === Hand.Left ? this.endPosition.leftArmEnd : this.endPosition.rightArmEnd;
      case "Linear":
        return super.interpolateHandOffset(progress, hand);
      case "Center":
        const position = this.interpolateOffset(progress);
        const offset = {
          x: position.x - this.center.x,
          y: position.y - this.center.y,
        };
        const rotation = degreesToRadians(this.interpolateRotation(progress));

        const centerPos = {
          x: -Math.cos(rotation) * offset.x + Math.sin(rotation) * offset.y,
          y: -Math.sin(rotation) * offset.x - Math.cos(rotation) * offset.y,
        }
        return centerPos;
      case "None":
        return hand === Hand.Left ? leftShoulder : rightShoulder;
    }
  }

  override drawDebug(ctx: CanvasRenderingContext2D, progress: number) {
    ctx.beginPath();
    const distance = this.closer?.middleDistance ?? this.endDistance;
    let start: number, end: number;
    if (Math.abs(this.endRotation - this.startRotation) >= 360) {
      start = 0;
      end = 2 * Math.PI;
    } else {
      start = degreesToRadians(this.startRotation);
      end = degreesToRadians(this.endRotation);
    }
    ctx.ellipse(this.center.x, this.center.y,
                this.xRadius * distance, this.yRadius * distance,
                0,
                start, end,
                this.endRotation < this.startRotation);
    ctx.stroke();
  }
}

// For "PassBy" and related moves where stepping straight forward would walk into the
//  other dancer, so instead move in thin oval
export class StepWideLinearAnimationSegment extends AnimationSegment {
  private readonly distanceAtMidpoint: number;
  private readonly sidewaysVector: Offset;
  private readonly movementAngle: number;
  private readonly handTransitionProgress: number;
  private readonly progressCenter?: number;
  private readonly actualCenter: Offset;
  private readonly hands: Map<Hand, HandAnimation>;
  private readonly facing: "Start" | "Forward";
  private readonly midPoint: Offset;

  constructor({ beats, startPosition, endPosition, distanceAtMidpoint, otherPath, hands, facing }: {
    beats: number;
    startPosition: DancerSetPosition;
    endPosition: DancerSetPosition;
    distanceAtMidpoint: number;
    otherPath?: { start: Offset, end: Offset };
    hands?: Map<Hand, HandAnimation>;
    facing?: "Start" | "Forward";
  }) {
    super(beats, startPosition, endPosition);

    this.hands = new Map<Hand, HandAnimation>(
      [Hand.Left, Hand.Right].map(h => [h, hands?.get(h) ?? { kind: "Linear" }]));
    this.facing = facing ?? "Start";

    this.distanceAtMidpoint = distanceAtMidpoint;

    const vector = OffsetMinus(this.endPosition.position, this.startPosition.position);
    const norm = vector.x * vector.x + vector.y * vector.y;
    const distance = Math.sqrt(norm);
    const sideways = OffsetRotate(vector, Rotation.Left);
    const normalizedVector = OffsetTimes(sideways, 1/distance);
    this.sidewaysVector = normalizedVector;

    this.movementAngle = radiansToDegrees(-Math.atan2(vector.y, vector.x)) + 90;

    // Center is the point where the dancer is closest to the other dancer.
    //  If omitted, it's assumed the movement is symmetrical, so that happens halfway through the move.
    //  Otherwise, find the point on the line between the start and end closest to the center.
    this.actualCenter = OffsetPlus(this.startPosition.position, OffsetTimes(vector, 0.5));
    if (otherPath) {
      /*
(xa(t), ya(t))
(xb(t), yb(t))

(xa_0 + xa_m*t, ya_0, ya_m*t)
(xb_0 + xb_m*t, yb_0, yb_m*t)


// can omit the sqrt
MIN[((xa_0 + xa_m*t) - (xb_0 + xb_m*t))**2 + ((ya_0, ya_m*t) - (yb_0, yb_m*t))**2]
MIN[(xa_0-xb_0 + (xa_m-xb_m)*t)**2 + (ya_0-yb_0 + (ya_m-yb_m)*t)**2]
MIN[(x_0 + x_m*t)**2 + (y_0 + y_m*t)**2]
MIN[(x_0**2 + 2x_0*x_m*t + x_m**2*t**2) + (y_0**2 + 2y_0*y_m*t + y_m**2*t**2)]
MIN[(x_0**2+y_0**2 + (2x_0*x_m+2y_0*y_m)*t + (x_m**2+y_m**2)*t**2)]

a = x_m**2+y_m**2 = (xa_m-xb_m)**2 + (ya_m-yb_m)**2
b = (2x_0*x_m+2y_0*y_m) = 2*((xa_0-xb_0)*(xa_m-xb_m) + (ya_0-yb_0)*(ya_m-yb_m))

t = -b/2a
       */
      const otherVector = OffsetMinus(otherPath.end, otherPath.start);
      const m = OffsetMinus(otherVector, vector);
      const i = OffsetMinus(otherPath.start, this.startPosition.position);

      const a = m.x*m.x + m.y*m.y;
      const b = 2*(i.x*m.x + i.y*m.y);

      const tMin = -b/(2*a);
      const t = tMin >= 0 && tMin <= 1 ? tMin : undefined;

      if (t !== undefined) {
        this.progressCenter = t;
        this.actualCenter = interpolateLinearOffset(t, this.startPosition.position, this.endPosition.position);
        const otherCenter = interpolateLinearOffset(t, otherPath.start, otherPath.end);
        const otherSideways = OffsetMinus(this.actualCenter, otherCenter);
        const sidewaysLinedUp = Math.sign(otherSideways.x) === Math.sign(sideways.x) && Math.sign(otherSideways.y) === Math.sign(sideways.y);
        const distanceAdjustment = OffsetDistance(this.actualCenter, otherCenter) * (sidewaysLinedUp ? +1 : -1);
        if (this.distanceAtMidpoint > 0) {
          this.distanceAtMidpoint = Math.max(0, this.distanceAtMidpoint - distanceAdjustment);
        } else {
          this.distanceAtMidpoint = Math.min(0, this.distanceAtMidpoint + distanceAdjustment);
        }
      }
    }

    this.handTransitionProgress = 0.5 / beats;

    this.midPoint = this.interpolateOffset(this.progressCenter ?? 0.5);
  }

  interpolateOffset(progress: number): Offset {
    let sidewaysProgress = progress;
    if (this.progressCenter) {
      if (progress === this.progressCenter) {
        sidewaysProgress = 0.5;
      } else if (progress < this.progressCenter) {
        sidewaysProgress = progress / this.progressCenter / 2;
      } else /* progress => this.progressCenter */ {
        sidewaysProgress = ((progress - this.progressCenter) / (1 - this.progressCenter)) / 2 + 0.5;
      }
    }

    return OffsetPlus(interpolateLinearOffset(progress, this.startPosition.position, this.endPosition.position),
      OffsetTimes(this.sidewaysVector, this.distanceAtMidpoint * Math.sin(sidewaysProgress * Math.PI)));
  }

  interpolateRotation(progress: number): number {
    if (this.facing === "Forward") {
      return this.movementAngle;
    } else /* "Start" */ {
      return this.startPosition.rotation;
    }
  }

  interpolateHandOffset(progress: number, hand: Hand): Offset | undefined {
    const progressCenter = this.progressCenter ?? 0.5;
    const handAnim : HandAnimation = this.hands.get(hand)!;
    switch (handAnim.kind) {
      case "Start":
        return hand === Hand.Left ? this.startPosition.leftArmEnd : this.startPosition.rightArmEnd;
      case "End":
        return hand === Hand.Left ? this.endPosition.leftArmEnd : this.endPosition.rightArmEnd;
      case "Linear":
        return super.interpolateHandOffset(progress, hand);
      case "CenterUntilPassed":
        if (progress > progressCenter) {
          return hand.shoulderPosition();
        }
      case "Center":
        const position = this.interpolateOffset(progress);
        const offset = {
          x: position.x - this.actualCenter.x,
          y: position.y - this.actualCenter.y,
        };
        const rotation = degreesToRadians(this.interpolateRotation(progress));

        const centerPos = {
          x: -Math.cos(rotation) * offset.x + Math.sin(rotation) * offset.y,
          y: -Math.sin(rotation) * offset.x - Math.cos(rotation) * offset.y,
        }

        if (handAnim.kind === "CenterUntilPassed" && progress + this.handTransitionProgress > progressCenter) {
          return interpolateLinearOffset((progressCenter - progress) / this.handTransitionProgress,
                                         hand.shoulderPosition(), centerPos);
        }
        return centerPos;
      case "None":
        return hand.shoulderPosition();
    }
  }

  override drawDebug(ctx: CanvasRenderingContext2D, progress: number) {
    ctx.beginPath();
    ctx.moveTo(this.startPosition.position.x, this.startPosition.position.y);
    ctx.lineTo(this.midPoint.x, this.midPoint.y);
    ctx.lineTo(this.endPosition.position.x, this.endPosition.position.y);
    ctx.stroke();

    ctx.beginPath();
    ctx.ellipse(this.actualCenter.x, this.actualCenter.y, 0.075, 0.075, 0, 0, 2*Math.PI);
    ctx.stroke();
  }
}

export class SlideAnimationSegment extends AnimationSegment {
  private readonly withoutSlide: AnimationSegment[];
  private readonly slideAmount: Offset;

  constructor(beats:number, withoutSlide:AnimationSegment[], slideAmount:Offset) {
    super(
      beats,
      withoutSlide[0].startPosition,
      {
        ...withoutSlide.at(-1)!.endPosition,
        position: OffsetPlus(withoutSlide.at(-1)!.endPosition.position, slideAmount)
      });

    this.withoutSlide = withoutSlide;
    this.slideAmount = slideAmount;
  }

  private selectSegment(progress: number): { segment: AnimationSegment, segmentProgress: number } {
    let beat = progress * this.beats;
    for (const segment of this.withoutSlide) {
      if (beat <= segment.beats) {
        return { segment, segmentProgress: beat / segment.beats };
      } else {
        beat -= segment.beats;
      }
    }

    return { segment: this.withoutSlide.at(-1)!, segmentProgress: 1 };
  }

  interpolateOffset(progress: number): Offset {
    const { segment, segmentProgress } = this.selectSegment(progress);
    return OffsetPlus(segment.interpolateOffset(segmentProgress), OffsetTimes(this.slideAmount, progress));
  }

  interpolateHandOffset(progress: number, hand: Hand): Offset | undefined {
    const { segment, segmentProgress } = this.selectSegment(progress);
    return segment.interpolateHandOffset(segmentProgress, hand);
  }

  interpolateRotation(progress: number): number {
    const { segment, segmentProgress } = this.selectSegment(progress);
    return segment.interpolateRotation(segmentProgress);
  }

  override drawDebug(ctx: CanvasRenderingContext2D, progress: number) {
    // TODO Right way to handle the slide part?
    const slide = OffsetTimes(this.slideAmount, progress);
    ctx.translate(slide.x, slide.y);

    const { segment, segmentProgress } = this.selectSegment(progress);
    segment.drawDebug(ctx, segmentProgress);
  }
}

export class Animation {
  private readonly segments : Map<DancerIdentity, AnimationSegment[]>;
  public readonly progression : Offset;
  public readonly numBeats : number;
  public readonly progressionError : string | undefined;

  constructor(segments: Map<DancerIdentity, AnimationSegment[]>) {
    this.segments = segments;
    this.numBeats = Math.max(...[...this.segments.values()].map(el => el.reduce((a, s) => a + s.beats, 0)));

    const progressions = new Map<DancerIdentity, Offset>(
      [...this.segments.entries()]
        .map(([id, a]) => [
          id,
          OffsetMinus(
            a.at(-1)!.endPosition.position,
            a.at(0)!.startPosition.position)]));
    this.progression = progressions.get(DancerIdentity.OnesLark)!;
    if (this.progression.x !== 0) {
      this.progressionError = "Progressing to different line is unsupported.";
    }
    if (this.progression.y === 0) {
      this.progressionError = "Dance does not progress.";
    }
    if (!OffsetEquals(progressions.get(DancerIdentity.OnesLark)!, progressions.get(DancerIdentity.OnesRobin)!)) {
      this.progressionError = "Ones are not progressing the same as each other.";
    }
    if (!OffsetEquals(progressions.get(DancerIdentity.TwosLark)!, progressions.get(DancerIdentity.TwosRobin)!)) {
      this.progressionError = "Twos are not progressing the same as each other.";
    }
    if (!OffsetEquals(progressions.get(DancerIdentity.OnesLark)!, OffsetMinus(offsetZero, progressions.get(DancerIdentity.TwosLark)!))) {
      this.progressionError = "Ones and Twos are not progressing opposite each other.";
    }
    if (this.progression.y % setHeight / 2 !== 0) {
      this.progressionError = "Progression is not an integer number of places.";
    }
  }

  positionsAtBeat(beat: number, progression?: number): DancersSetPositions {
    function positionAtBeat(animation: AnimationSegment[], beat: number): DancerSetPosition {
      let lastFrame: AnimationSegment = animation[0];
      let lastFrameEndBeat = 0;
      for (const frame of animation) {
        let currentFrameEndBeat = lastFrameEndBeat + frame.beats;
        if (currentFrameEndBeat < beat) {
          lastFrame = frame;
          lastFrameEndBeat = currentFrameEndBeat;
          continue;
        } else if (currentFrameEndBeat === beat) {
          return frame.endPosition;
        } else {
          const progress = (beat - lastFrameEndBeat) / (currentFrameEndBeat - lastFrameEndBeat);
          return frame.positionAtFraction(progress);
        }
      }

      // fallthrough
      return lastFrame.endPosition;
    }

    progression ??= 0;
    while (beat < 0) {
      beat += this.numBeats;
      progression -= 1;
    }
    while (beat > this.numBeats) {
      beat -= this.numBeats;
      progression += 1;
    }
    const res = new Map<DancerIdentity, DancerSetPosition>();
    for (const [id, animationSegments] of this.segments.entries()) {
      const basePosition = positionAtBeat(animationSegments, beat);
      if (progression !== 0) {
        const progressionOffset = OffsetTimes(
          this.progression,
          progression * (id.coupleRole === CoupleRole.Ones ? 1 : -1));
        res.set(id, {
          ...basePosition,
          position: OffsetPlus(
            basePosition.position,
            progressionOffset),
          drawDebug: basePosition.drawDebug ?
            (ctx) => { ctx.translate(progressionOffset.x, progressionOffset.y); basePosition.drawDebug!(ctx); }
            : undefined
        });
      } else {
        res.set(id, basePosition);
      }
    }
    return res;
  }
}