Vectorize xml_find_* helpers

[MichaelChirico]

In {lintr}, I found that a natural replacement for the common idiom xml_text(xml_find_first(x, "XPATH")) is xml_find_chr(x, "string(XPATH)"). This already dramatically reduces the memory footprint, but the R-side looping kills the performance a bit (e.g. 30% slower).

This PR proposes vectorizing the whole operation in C(++) instead, providing 10-40x performance improvement and 50-75% memory reduction.

PR prepared by Gemini. There are a number of similar ways to implement the proposal -- filing first to probe interest in maintaining this approach before attempting to fine-tune the details.

Given the apparent "correctness" of using xml_find_chr(), I will be changing {lintr} to use it anyway -- it would be nice to get the free performance enhancement, too.

Benchmark Results

Medium Nodeset ($N = 500$ items)

Expression	Median Time	Iterations/sec	Memory Allocated	Speedup vs Baseline
xml_text(xml_find_first(x, 'name'))	3.85 ms	254	7.91 KB	1.0x (baseline)
xml_find_chr(x, 'string(name)') (OLD vapply)	4.55 ms	213	3.95 KB	0.85x
xml_find_chr(x, 'string(name)') (NEW C++)	243.9 µs	3,903	3.95 KB	~15.8x faster
!is.na(xml_find_first(x, 'active[...]'))	5.09 ms	194	7.95 KB	1.0x (baseline)
xml_find_lgl(x, 'boolean(...)') (OLD vapply)	4.24 ms	231	2.00 KB	1.2x
xml_find_lgl(x, 'boolean(...)') (NEW C++)	312.0 µs	3,071	2.00 KB	~16.3x faster
xml_find_num(x, 'number(...)') (OLD vapply)	4.25 ms	231	3.95 KB	1.0x
xml_find_num(x, 'number(...)') (NEW C++)	233.2 µs	4,175	3.95 KB	~18.2x faster
xml_find_int(x, 'count(...)') (OLD vapply)	4.09 ms	240	2.00 KB	1.0x
xml_find_int(x, 'count(...)') (NEW C++)	192.4 µs	5,032	2.00 KB	~21.3x faster

---

Large Nodeset ($N = 5{,}000$ items)

Expression	Median Time	Iterations/sec	Memory Allocated	Speedup vs Baseline
xml_text(xml_find_first(x, 'name'))	50.97 ms	17.8	78.2 KB	1.0x (baseline)
xml_find_chr(x, 'string(name)') (OLD vapply)	73.16 ms	13.5	39.1 KB	0.70x
xml_find_chr(x, 'string(name)') (NEW C++)	1.88 ms	432	39.1 KB	~27.1x faster (vs xml_text) ~38.9x faster (vs old xml_find_chr)
!is.na(xml_find_first(x, 'active[...]'))	79.80 ms	13.2	78.3 KB	1.0x (baseline)
xml_find_lgl(x, 'boolean(...)') (OLD vapply)	65.00 ms	15.0	19.6 KB	1.2x
xml_find_lgl(x, 'boolean(...)') (NEW C++)	2.60 ms	371	19.6 KB	~30.7x faster (vs !is.na) ~25.0x faster (vs old xml_find_lgl)
xml_find_num(x, 'number(...)') (OLD vapply)	42.66 ms	23.4	39.1 KB	1.0x
xml_find_num(x, 'number(...)') (NEW C++)	1.82 ms	511	39.1 KB	~23.4x faster
xml_find_int(x, 'count(...)') (OLD vapply)	39.75 ms	25.2	19.6 KB	1.0x
xml_find_int(x, 'count(...)') (NEW C++)	1.44 ms	656	19.6 KB	~27.6x faster

Benchmark script

library(xml2)
library(bench)

# Generate a synthetic XML catalog of N records
generate_xml <- function(n) {
  items <- sprintf(
    "<item id=\"%d\"><name>Product_%d</name><price>%.2f</price><stock>%d</stock><active>%s</active></item>",
    seq_len(n), seq_len(n), seq_len(n) * 1.5, seq_len(n) * 10L, ifelse(seq_len(n) %% 2 == 0, "true", "false")
  )
  xml_str <- paste0("<catalog>\n", paste(items, collapse = "\n"), "\n</catalog>")
  read_xml(xml_str)
}

# Baseline vapply implementations in xml2 <= 1.6.0
old_find_chr <- function(x, xpath, ns = xml_ns(x)) {
  vapply(x, \(x) xml_find_chr.xml_node(x, xpath = xpath, ns = ns), character(1))
}
old_find_lgl <- function(x, xpath, ns = xml_ns(x)) {
  vapply(x, \(x) xml_find_lgl.xml_node(x, xpath = xpath, ns = ns), logical(1))
}
old_find_num <- function(x, xpath, ns = xml_ns(x)) {
  vapply(x, \(x) xml_find_num.xml_node(x, xpath = xpath, ns = ns), numeric(1))
}
old_find_int <- function(x, xpath, ns = xml_ns(x)) {
  vapply(x, \(x) xml_find_int.xml_node(x, xpath = xpath, ns = ns), integer(1))
}

for (n in c(50, 500, 5000, 50000, 500000)) {
  run_bench = function(...) {
    print(bench::mark(..., iterations = if (n >= 5000) 30 else 100, check = TRUE))
  }
  doc <- generate_xml(n)
  items <- xml_find_all(doc, ".//item")

  # 1. String Extraction
  run_bench(
    `xml_text(xml_find_first(x, 'name'))`          = xml_text(xml_find_first(items, "name")),
    `xml_find_chr(x, 'string(name)') [OLD vapply]` = old_find_chr(items, "string(name)"),
    `xml_find_chr(x, 'string(name)') [NEW C++]`    = xml_find_chr(items, "string(name)")
  )

  # 2. Boolean Existence Query
  run_bench(
    `!is.na(xml_find_first(x, 'active[...]'))`      = !is.na(xml_find_first(items, "active[text()='true']")),
    `xml_find_lgl(x, 'boolean(...)') [OLD vapply]` = old_find_lgl(items, "boolean(active[text()='true'])"),
    `xml_find_lgl(x, 'boolean(...)') [NEW C++]`    = xml_find_lgl(items, "boolean(active[text()='true'])")
  )

  # 3. Numeric Extraction
  run_bench(
    `xml_find_num(x, 'number(price)') [OLD vapply]` = old_find_num(items, "number(price)"),
    `xml_find_num(x, 'number(price)') [NEW C++]`    = xml_find_num(items, "number(price)")
 )
  run_bench(
    `xml_find_int(x, 'count(*)') [OLD vapply]`      = old_find_int(items, "count(*)"),
    `xml_find_int(x, 'count(*)') [NEW C++]`         = xml_find_int(items, "count(*)")
  )
}