add str_insert() to insert a string after the i-th character of anoth…

…er string; this should be faster than str_replace() for insert_header_[latex|html]

R/header.R

@@ -56,8 +56,7 @@ insert_header_latex = function(doc, b) {
       doc[j] = sub(p, '\n\\\\IfFileExists{upquote.sty}{\\\\usepackage{upquote}}{}\n\\2', doc[j], perl = TRUE)
     }
     i = i[1L]; l = str_locate(doc[i], b, FALSE)
-    tmp = substr(doc[i], l[, 1], l[, 2])
-    doc[i] = str_replace(doc[i], l, paste0(tmp, make_header_latex(doc)))
+    doc[i] = str_insert(doc[i], l[, 2], make_header_latex(doc))
   } else if (parent_mode() && !child_mode()) {
     # in parent mode, we fill doc to be a complete document
     doc[1L] = one_string(c(
@@ -87,8 +86,7 @@ insert_header_html = function(doc, b) {
   i = grep(b, doc)
   if (length(i) == 1L) {
     l = str_locate(doc[i], b, FALSE)
-    tmp = substr(doc[i], l[, 1], l[, 2])
-    doc[i] = str_replace(doc[i], l, paste0(tmp, '\n', make_header_html()))
+    doc[i] = str_insert(doc[i], l[, 2], paste0('\n', make_header_html()))
   }
   doc
 }

R/utils-string.R

@@ -9,6 +9,14 @@ str_replace = function(x, pos, value) {
   paste(rbind(y, c(value, '')), collapse = '')
 }
 
+# insert a value after i-th character in the string x
+str_insert = function(x, i, value) {
+  if (i <= 0) return(paste0(value, x))
+  n = nchar(x)
+  if (n == 0 || i >= n) return(paste0(x, value))
+  paste0(substr(x, 1, i), value, substr(x, i + 1, n))
+}
+
 # a wrapper function to make strwrap() return a character vector of the same
 # length as the input vector; each element of the output vector is a string
 # formed by concatenating wrapped strings by \n