R version 4.5.0 RC (2025-04-03 r88103 ucrt) -- "How About a Twenty-Six"
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Platform: x86_64-w64-mingw32/x64

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> if(FALSE) {  # This works but takes a lot of time. So leave this out
+              # for default runs.
+ library(RJSONIO)
+ 
+ v = rpois(100000, 4)
+ x = toJSON(v)
+ 
+  # 0.054  seconds
+ system.time({ val = fromJSON(x, getNativeSymbolInfo("R_json_IntegerArrayCallback"),
+                               data = rep(as.integer(NA), 100000))})
+ 
+ all(v == val)
+ 
+ # 89.981 So a factor of 1666
+ system.time({v1 = fromJSON(x)})
+ 
+ 
+ # Now if we preallocate a vector with the size of the result
+ # and fill it with an R callback, we get 1.050.
+ # This is a factor of 20 relative to the specialized C code.
+ gen =
+ function(n = 100000, ans = integer(n))
+ {
+   pos = 1
+   update =
+   function(type, val) {
+     if(type == JSON_T_INTEGER) {
+       ans[pos] <<- val
+       pos <<- pos + 1
+     }
+     TRUE
+   }
+   list(update = update, value = function() ans)
+ }
+ h = gen()
+ system.time({v1 = fromJSON(x, h$update)})
+ 
+ 
+ # Now let's use the generic list
+ # We get 1.130
+ 
+ gen.list =
+ function(n = 100000, ans = vector("list", n))
+ {
+   pos = 1
+   update =
+   function(type, val) {
+     if(type == JSON_T_INTEGER) {
+       ans[pos] <<- val
+       pos <<- pos + 1
+     }
+     TRUE
+   }
+   structure(list(update = update, value = function() unlist(ans[1:pos])),
+              class = "JSONParserHandler")
+ }
+ h = gen.list()
+ system.time({v1 = fromJSON(x, h)})
+ 
+ 
+ #
+ gen.list =
+ function(n = 100000, ans = vector("list", n))
+ {
+   pos = 1
+   update =
+   function(type, val) {
+     if(type == JSON_T_INTEGER) {
+       if(length(ans) == pos)
+         length(ans) <<- 2*pos
+       ans[pos] <<- val
+       pos <<- pos + 1
+     }
+     TRUE
+   }
+   structure(list(update = update, value = function() unlist(ans[1:pos])),
+              class = "JSONParserHandler")
+ }
+ h = gen.list()
+ system.time({v1 = fromJSON(x, h)})
+ 
+ 
+ 
+ ############################################
+ 
+ v = rpois(100000, 4)
+ x = toJSON(v)
+ 
+ h = RJSONIO:::simpleJSONHandler()
+ system.time({a = fromJSON(x, h)})
+ #   user  system elapsed 
+ # 59.470  21.693  91.442 
+ system.time({a = fromJSON(x)})
+ #   user  system elapsed 
+ #  3.899   0.117   4.415 
+ system.time({a = fromJSON(x, default.size = 10000)})
+ #   user  system elapsed 
+ #  3.957   0.116   4.465 
+ system.time({a = fromJSON(x, default.size = 100000)})
+ }
> 
> proc.time()
   user  system elapsed 
   0.15    0.01    0.15