add first and last reducers for window (#636)

Author: mbostock

README.md

@@ -1463,6 +1463,8 @@ The following window reducers are supported:
 * *variance* - the variance per [Welford’s algorithm](https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm)
 * *difference* - the difference between the last and first window value
 * *ratio* - the ratio of the last and first window value
+* *first* - the first value
+* *last* - the last value
 * a function to be passed an array of *k* values
 
 By default, **anchor** is *middle* and **reduce** is *mean*.

src/transforms/window.js

@@ -51,6 +51,8 @@ function maybeReduce(reduce = "mean") {
       case "variance": return reduceSubarray(variance);
       case "difference": return reduceDifference;
       case "ratio": return reduceRatio;
+      case "first": return reduceFirst;
+      case "last": return reduceLast;
     }
   }
   if (typeof reduce !== "function") throw new Error("invalid reduce");
@@ -130,3 +132,23 @@ function reduceRatio(k, s) {
     }
   };
 }
+
+function reduceFirst(k, s) {
+  return {
+    map(I, S, T) {
+      for (let i = 0, n = I.length - k; i < n; ++i) {
+        T[I[i + s]] = S[I[i]];
+      }
+    }
+  };
+}
+
+function reduceLast(k, s) {
+  return {
+    map(I, S, T) {
+      for (let i = 0, n = I.length - k; i < n; ++i) {
+        T[I[i + s]] = S[I[i + k - 1]];
+      }
+    }
+  };
+}