Tracking the API of a CRAN package over time

API.R

if (!require(httr)) {
  devtools::install_github("httr")
  library(httr)
}

get_archived_packages <- function(package, repo, where) {
  archive <- file.path(repo, "Archive", package)
  response <- httr::GET(archive)
  if (response$status_code == "404")
    stop("Could not access page at '", archive, "'.")
  content <- httr::content(response, "text")
  matches <- gregexpr("<a href=\"(.*?)\">", content, perl = TRUE)
  starts <- as.integer(matches[[1]])
  ends <- starts + attr(matches[[1]], "match.length")
  links <- substring(content, starts, ends)
  tarballs <- unlist(lapply(strsplit(links, '"', fixed = TRUE), "[[", 2))
  tarballs <- grep(package, tarballs, value = TRUE)
  links <- file.path(archive, tarballs)
  lapply(links, function(link) {
    download.file(link, destfile = file.path(where, basename(link)))
  })
}

API <- function(package,
                repo = contrib.url(getOption("repos"), type = "source"),
                where = file.path(
                  normalizePath(tempdir(), winslash = "/"),
                  paste(package, "API", sep = "-")
                )
) {
  
  unloadNamespace(package)
  dir.create(where, recursive = TRUE, showWarnings = FALSE)
  get_archived_packages(package, repo, where)
  tarballs <- list.files(where, full.names = TRUE)
  templib <- path.expand(file.path("~", ".devtools", package, "library"))
  dir.create(templib, recursive = TRUE, showWarnings = FALSE)
  api <- lapply(tarballs, function(tarball) {
    install.packages(tarball, lib = templib, repos = NULL, quiet = TRUE, type = "source")
    library(package, lib.loc = templib, character.only = TRUE)
    exports <- getNamespaceExports(package)
    result <- lapply(exports, function(export) {
      formals(export)
    })
    names(result) <- exports
    unloadNamespace(package)
    result
  })
  names(api) <- gsub("\\.tar\\.gz$*", "", basename(tarballs))
  class(api) <- "API"
  api
}

history <- function(api) {
  all_names <- sort(unique(unlist(lapply(api, names))))
  result <- do.call(rbind, lapply(api, function(x) {
    all_names %in% names(x)
  }))
  colnames(result) <- all_names
  result
}

devtools_api <- API("devtools")
history(devtools_api)