only comparator if function.length === 2 (#238)

* only comparator if function.length === 2

* only comparator if function.length === 2

* only comparator if function.length === 2

* only comparator if function.length === 2

* import ascending

* relax error message test

* Update README

README.md

@@ -543,7 +543,7 @@ For descending order, negate the group value:
 d3.groupSort(barley, g => -d3.median(g, d => d.yield), d => d.variety)
 ```
 
-If a *comparator* is passed instead of an *accessor* (i.e., if the second argument is a function that takes two arguments), it will be asked to compare two groups *a* and *b* and should return a negative value if *a* should be before *b*, a positive value if *a* should be after *b*, or zero for a partial ordering.
+If a *comparator* is passed instead of an *accessor* (i.e., if the second argument is a function that takes exactly two arguments), it will be asked to compare two groups *a* and *b* and should return a negative value if *a* should be before *b*, a positive value if *a* should be after *b*, or zero for a partial ordering.
 
 <a name="count" href="#count">#</a> d3.<b>count</b>(<i>iterable</i>[, <i>accessor</i>]) · [Source](https://github.com/d3/d3-array/blob/main/src/count.js), [Examples](https://observablehq.com/@d3/d3-count)
 
@@ -744,7 +744,7 @@ Returns an array containing the values in the given *iterable* in the sorted ord
 d3.sort(new Set([0, 2, 3, 1])) // [0, 1, 2, 3]
 ```
 
-If an *accessor* (a function that takes a single argument) is specified,
+If an *accessor* (a function that does not take exactly two arguments) is specified,
 
 ```js
 d3.sort(data, d => d.value)

src/bisector.js

@@ -5,7 +5,7 @@ export default function bisector(f) {
   let compare1 = f;
   let compare2 = f;
 
-  if (f.length === 1) {
+  if (f.length !== 2) {
     delta = (d, x) => f(d) - x;
     compare1 = ascending;
     compare2 = (d, x) => ascending(f(d), x);

src/groupSort.js

@@ -3,7 +3,7 @@ import group, {rollup} from "./group.js";
 import sort from "./sort.js";
 
 export default function groupSort(values, reduce, key) {
-  return (reduce.length === 1
+  return (reduce.length !== 2
     ? sort(rollup(values, reduce, key), (([ak, av], [bk, bv]) => ascending(av, bv) || ascending(ak, bk)))
     : sort(group(values, key), (([ak, av], [bk, bv]) => reduce(av, bv) || ascending(ak, bk))))
     .map(([key]) => key);

src/rank.js

@@ -5,7 +5,7 @@ export default function rank(values, valueof = ascending) {
   if (typeof values[Symbol.iterator] !== "function") throw new TypeError("values is not iterable");
   let V = Array.from(values);
   const R = new Float64Array(V.length);
-  if (valueof.length === 1) V = V.map(valueof), valueof = ascending;
+  if (valueof.length !== 2) V = V.map(valueof), valueof = ascending;
   const compareIndex = (i, j) => valueof(V[i], V[j]);
   let k, r;
   Uint32Array

src/sort.js

@@ -1,10 +1,11 @@
+import ascending from "./ascending.js";
 import permute from "./permute.js";
 
 export default function sort(values, ...F) {
   if (typeof values[Symbol.iterator] !== "function") throw new TypeError("values is not iterable");
   values = Array.from(values);
   let [f] = F;
-  if ((f && f.length === 1) || F.length > 1) {
+  if ((f && f.length !== 2) || F.length > 1) {
     const index = Uint32Array.from(values, (d, i) => i);
     if (F.length > 1) {
       F = F.map(f => values.map(f));
@@ -20,10 +21,11 @@ export default function sort(values, ...F) {
     }
     return permute(values, index);
   }
-  return values.sort(f === undefined ? ascendingDefined : compareDefined(f));
+  return values.sort(compareDefined(f));
 }
 
-export function compareDefined(compare) {
+export function compareDefined(compare = ascending) {
+  if (compare === ascending) return ascendingDefined;
   if (typeof compare !== "function") throw new TypeError("compare is not a function");
   return (a, b) => {
     const x = compare(a, b);

test/sort-test.js

@@ -61,12 +61,12 @@ it("sort(values) accepts an iterable", () => {
 });
 
 it("sort(values) enforces that values is iterable", () => {
-  assert.throws(() => sort({}), {name: "TypeError", message: "values is not iterable"});
+  assert.throws(() => sort({}), {name: "TypeError", message: /is not iterable/});
 });
 
 it("sort(values, comparator) enforces that comparator is a function", () => {
-  assert.throws(() => sort([], {}), {name: "TypeError", message: "compare is not a function"});
-  assert.throws(() => sort([], null), {name: "TypeError", message: "compare is not a function"});
+  assert.throws(() => sort([], {}), {name: "TypeError", message: /is not a function/});
+  assert.throws(() => sort([], null), {name: "TypeError", message: /is not a function/});
 });
 
 it("sort(values) does not skip sparse elements", () => {