/**
 * Line and area path generators. Inputs are already in pixel space (the caller
 * maps data through scales first), so these are pure string builders.
 */

import type { AreaPoint, Curve, Point } from './types'
import { coord } from './path'
import { curveLinear } from './curve'

export interface LineOptions {
	/** How to connect points. Default: straight segments. */
	curve?: Curve
}

/**
 * An open polyline through `points`. Returns an empty string for no points (a
 * valid, invisible `d`), so binding it to a `<path>` is always safe.
 */
export function line(points: readonly Point[], options: LineOptions = {}): string {
	if (points.length === 0) return ''
	const link = options.curve ?? curveLinear
	return `M${coord(points[0].x, points[0].y)}${link(points)}`
}

export interface AreaOptions {
	curve?: Curve
}

/**
 * A filled band between the `y0` (baseline) and `y1` (value) edges at each `x`.
 * The outline runs along the top edge left→right, then back along the baseline
 * right→left, and closes — so the same curve shapes both edges.
 */
export function area(points: readonly AreaPoint[], options: AreaOptions = {}): string {
	if (points.length === 0) return ''
	const link = options.curve ?? curveLinear
	const tops: Point[] = points.map((p) => ({ x: p.x, y: p.y1 }))
	const baseline: Point[] = points.map((p) => ({ x: p.x, y: p.y0 })).reverse()
	return (
		`M${coord(tops[0].x, tops[0].y)}${link(tops)}` +
		`L${coord(baseline[0].x, baseline[0].y)}${link(baseline)}Z`
	)
}