window transform: difference, ratio, first and last drop the trailing outputs of each series

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技术栈
javascript, node.js
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data-visualization, frontend

调研方向

阅读 src/transforms/window.js 并检查 reduceDifference、reduceRatio、reduceFirst 和 reduceLast，将它们在 strict 和 non-strict 模式下的循环边界与示例中描述的行为进行比较。为两种模式下的末尾输出添加或更新测试，然后运行相关的 window transform 测试。当这些 reducer 生成预期的完整长度输出（包括 strict 为 false 时被截短的末尾窗口）时，即表示完成。

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描述

The difference, ratio, first and last window reducers stop their loops early: with strict: true the last full window is missing, and with strict: false the last k - 1 - s truncated windows are missing. Other reducers such as mean fill these positions.

import * as Plot from "@observablehq/plot";

const run = (w, data) => {
  const o = Plot.windowY(w, {x: "i", y: "v"});
  o.transform(data, [Uint32Array.from(data, (_, i) => i)]);
  return Array.from(o.y.transform());
};
const d = [2, 4, 8, 16].map((v, i) => ({i, v}));

console.log(run({k: 2, reduce: "difference", strict: true, anchor: "end"}, d));
console.log(run({k: 2, reduce: "ratio",      strict: true, anchor: "end"}, d));
console.log(run({k: 1, reduce: "last",       strict: true}, d));
console.log(run({k: 3, reduce: "first",      anchor: "start"}, d));
console.log(run({k: 3, reduce: "difference", anchor: "start"}, d));
console.log(run({k: 3, reduce: "mean",       anchor: "start"}, d));

node t.mjs

Actual:

[ undefined, 2, 4, undefined ]
[ undefined, 2, 2, undefined ]
[ 2, 4, 8, undefined ]
[ 2, 4, undefined, undefined ]
[ 6, 12, undefined, undefined ]
[ 4.666666666666667, 9.333333333333334, 12, 16 ]

Expected:

[ undefined, 2, 4, 8 ]
[ undefined, 2, 2, 2 ]
[ 2, 4, 8, 16 ]
[ 2, 4, 8, 16 ]
[ 6, 12, 8, 0 ]
[ 4.666666666666667, 9.333333333333334, 12, 16 ]

In a chart, Plot.lineY(data, Plot.windowY({k: 7, reduce: "difference"}, ...)) stops short of the right edge.

Tested with @observablehq/plot 0.6.19 and main 2245188, Node.js v26.13.0.

In src/transforms/window.js, reduceDifference, reduceRatio, reduceFirst and reduceLast use n = I.length - k in the strict branch (should be I.length - k + 1) and n = I.length - k + s + 1 in the non-strict branch (should be I.length - s, with the window clipped to I.length).

Credit: https://ucsbplse.github.io/

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