Add lua scripts for generating n-grams from SKPs

tools/lua/ngrams.lua

+-- Generate n-grams of Skia API calls from SKPs.
+
+-- To test this locally, run:
+-- $ GYP_DEFINES="skia_shared_lib=1" make lua_pictures
+-- $ out/Debug/lua_pictures -q -r $SKP_DIR -l tools/lua/ngrams.lua > /tmp/lua-output && lua tools/lua/ngrams_aggregate.lua
+
+-- To run on Cluster Telemetry, copy and paste the contents of this file into
+-- the box at https://skia-tree-status.appspot.com/skia-telemetry/lua_script,
+-- and paste the contents of ngrams_aggregate.lua into the "aggregator script"
+-- box on the same page.
+
+-- Change n as desired.
+local n = 3
+
+-- This algorithm uses a list-of-lists for each SKP. For API call, append a
+-- list containing just the verb to the master list. Then, backtrack over the
+-- last (n-1) sublists in the master list and append the verb to those
+-- sublists. At the end of execution, the master list contains a sublist for
+-- every verb in the SKP file. Each sublist has length n, with the exception of
+-- the last n-1 sublists, which are discarded in the summarize() function,
+-- which generates counts for each n-gram.
+
+local ngrams = {}
+local currentFile = ""
+
+function sk_scrape_startcanvas(c, fileName)
+  currentFile = fileName
+  ngrams[currentFile] = {}
+end
+
+function sk_scrape_endcanvas(c, fileName)
+end
+
+function sk_scrape_accumulate(t)
+  table.insert(ngrams[currentFile], {t.verb})
+  for i = 1, n-1 do
+    local idx = #ngrams[currentFile] - i
+    if idx > 0 then
+      table.insert(ngrams[currentFile][idx], t.verb)
+    end
+  end
+end
+
+function sk_scrape_summarize()
+  -- Count the n-grams.
+  local counts = {}
+  for file, ngramsInFile in pairs(ngrams) do
+    for i = 1, #ngramsInFile - (n-1) do
+      local ngram = table.concat(ngramsInFile[i], " ")
+      if counts[ngram] == nil then
+        counts[ngram] = 1
+      else
+        counts[ngram] = counts[ngram] + 1
+      end
+    end
+  end
+
+  -- Write out code for aggregating.
+  for ngram, count in pairs(counts) do
+    io.write("if counts['", ngram, "'] == nil then counts['", ngram, "'] = ", count, " else counts['", ngram, "'] = counts['", ngram, "'] + ", count, " end\n")
+  end
+end