Improve performance of transpose_list()

R/transpose_list.R

@@ -1,4 +1,12 @@
 #' @useDynLib jsonlite C_transpose_list
 transpose_list <- function(x, names) {
-  .Call(C_transpose_list, x, names)
+  # Sort names before entering C, allowing for a binary search
+  LC_COLLATE <- "LC_COLLATE"
+  collate_before <- Sys.getlocale(LC_COLLATE)
+  on.exit(Sys.setlocale(LC_COLLATE, collate_before))
+  Sys.setlocale(LC_COLLATE, "C")
+  sorted_names <- sort(names)
+
+  transposed <- .Call(C_transpose_list, x, sorted_names)
+  transposed[match(names, sorted_names)]
 }

src/transpose_list.c

@@ -1,26 +1,55 @@
 #include <Rinternals.h>
 #include <string.h>
 
+// names is assumed to be sorted, to make names matching faster
+// by using a binary search
 SEXP C_transpose_list(SEXP x, SEXP names) {
   size_t ncol = Rf_length(names);
   size_t nrow = Rf_length(x);
   SEXP out = PROTECT(allocVector(VECSXP, ncol));
+
+  // Allocate output
   for(size_t i = 0; i < ncol; i++){
-    const char * targetname = CHAR(STRING_ELT(names, i));
     SEXP col = PROTECT(allocVector(VECSXP, nrow));
-    for(size_t j = 0; j < nrow; j++){
-      //search for 'targetname' in each record j
-      SEXP list = VECTOR_ELT(x, j);
-      SEXP listnames = getAttrib(list, R_NamesSymbol);
-      for(size_t k = 0; k < Rf_length(listnames); k++){
-        if(!strcmp(CHAR(STRING_ELT(listnames, k)), targetname)){
+    SET_VECTOR_ELT(out, i, col);
+    UNPROTECT(1);
+  }
+
+  // Find and save all elements in their transposed place
+  for(size_t j = 0; j < nrow; j++){
+    SEXP list = VECTOR_ELT(x, j);
+    SEXP listnames = getAttrib(list, R_NamesSymbol);
+    size_t listlength = Rf_length(listnames);
+
+    for(size_t k = 0; k < listlength; k++){
+      const char * listname = CHAR(STRING_ELT(listnames, k));
+
+      // Binary search for a name match
+      size_t low = 0;
+      size_t high = ncol - 1;
+      size_t mid;
+      while(1){
+        mid = (low + high) / 2;
+        const char * targetname = CHAR(STRING_ELT(names, mid));
+
+        int strcmp_result = strcmp(listname, targetname);
+        if(strcmp_result == 0){
+          // Match!
+          SEXP col = VECTOR_ELT(out, mid);
           SET_VECTOR_ELT(col, j, VECTOR_ELT(list, k));
           break;
+        } else if (strcmp_result > 0){
+          low = mid + 1;
+        } else {
+          high = mid - 1;
+        }
+
+        if (low > high) {
+          // No match to be found
+          break;
         }
       }
     }
-    SET_VECTOR_ELT(out, i, col);
-    UNPROTECT(1);
   }
   //setAttrib(out, R_NamesSymbol, names);
   UNPROTECT(1);

tests/testthat/test-simplifyDataFrame.R

@@ -0,0 +1,33 @@
+context("simplifyDataFrame")
+
+test_that("simplifyDataFrame() works", {
+  source <- list(
+    list(a = 11, b = 12),
+    list(d = 24),
+    list(a = 31, c = 33)
+  )
+
+  actual <- simplifyDataFrame(source, flatten = TRUE)
+
+  # Check that column order is preserved as discovered in the data
+  expect_equal(colnames(actual), c("a", "b", "d", "c"))
+
+  expect_row_equals <- function(number, expected) {
+    expect_equal(
+      as.numeric(actual[number, ]),
+      expected
+    )
+  }
+  #                       a   b   d   c
+  expect_row_equals(1, c(11, 12, NA, NA))
+  expect_row_equals(2, c(NA, NA, 24, NA))
+  expect_row_equals(3, c(31, NA, NA, 33))
+})
+
+test_that("transpose_list() does not change locale", {
+  locale_before <- Sys.getlocale()
+  transpose_list(list(a = 1), c("a"))
+  locale_after <- Sys.getlocale()
+
+  expect_equal(locale_before, locale_after)
+})