Vim: Occurrences: Add text previews

Files containing occurrences are read in a first step and lines
containing an occurrence are kept in memory.

Vim potentially has open buffers for some of these files but this
couldn't be used reliably as unused buffers would be emptied of their
content.
We don't open new buffers as that could trigger expensive function in
plugins and would use more memory.

vim/merlin/autoload/merlin.py

@@ -4,6 +4,7 @@
 import re
 import os
 import sys
+import itertools
 from sys import platform
 
 enclosing_types = [] # nothing to see here
@@ -495,6 +496,46 @@ def vim_document_at_cursor(path):
 def vim_document_under_cursor():
     vim_document_at_cursor(None)
 
+# Read the lines numbers present in [lines] from file [fname]. End-of-line is
+# stripped from each line.
+def read_lines_of_file(fname, lines):
+    lines = iter(sorted(set(lines)))
+    next_line = next(lines)
+    n = 1
+    r = {}
+    try:
+        try:
+            with open(fname) as inp:
+                for line in inp:
+                    if next_line == n:
+                        r[n] = line.rstrip("\n")
+                        next_line = next(lines)
+                    n += 1
+        except FileNotFoundError:
+            pass
+        # File has been truncated or not found
+        while True:
+            r[next_line] = ""
+            next_line = next(lines)
+    except StopIteration:
+        return r
+
+# From the result of [command_occurrences], read the start line for each
+# results. Generate the same occurrences with the 'text' field added.
+def with_text_previews(occurs):
+    preview_lines_by_file = {
+            fname: read_lines_of_file(fname, [ oc['start']['line'] for oc in occurs ])
+            for fname, occurs
+            in itertools.groupby(occurs, lambda oc: oc.get('file'))
+            if fname != None }
+    for oc in occurs:
+        lnum = oc['start']['line']
+        if 'file' not in oc: # Current buffer
+            text = vim.current.buffer[lnum - 1]
+        else:
+            text = preview_lines_by_file[oc['file']][lnum]
+        yield { "text": text, **oc }
+
 # Occurrences
 def vim_occurrences(vimvar, project_wide):
     vim.command("let %s = []" % vimvar)
@@ -504,21 +545,19 @@ def vim_occurrences(vimvar, project_wide):
     cur_fname = vim.current.buffer.name
     nr = 0
     cursorpos = 0
-    for pos in lst:
+    for pos in with_text_previews(lst):
         lnum = pos['start']['line']
         lcol = pos['start']['col']
         occur = { "lnum": lnum, "col": lcol + 1, "vcol": 0, "nr": nr,
-                 "pattern": "", "type": "I", "valid": 1 }
-        is_current_file = ('file' not in pos or pos['file'] == cur_fname)
-        if is_current_file:
-            occur["text"] = vim.current.buffer[lnum - 1]
+                 "pattern": "", "type": "I", "valid": 1 , "text": pos['text'] }
+        if 'file' not in pos or pos['file'] == cur_fname:
+            # Occurrence is in the current buffer
+            if (lnum, lcol) <= (line, col): cursorpos = nr
             occur["bufnr"] = bufnr
         else:
-            occur["text"] = ""
             occur["filename"] = pos['file']
         vim.command("let l:tmp = " + vim_record(occur))
         vim.command("call add(%s, l:tmp)" % vimvar)
-        if is_current_file and (lnum, lcol) <= (line, col): cursorpos = nr
         nr = nr + 1
     return cursorpos + 1