Add some unit tests for the BinarySearch function.

autoload/ale/cursor.vim

@@ -18,41 +18,6 @@ function! s:GetMessage(linter, type, text) abort
     return printf(l:msg, l:text)
 endfunction
 
-" This function will perform a binary search to find a message from the
-" loclist to echo when the cursor moves.
-function! s:BinarySearch(loclist, line, column) abort
-    let l:min = 0
-    let l:max = len(a:loclist) - 1
-    let l:last_column_match = -1
-
-    while 1
-        if l:max < l:min
-            return l:last_column_match
-        endif
-
-        let l:mid = (l:min + l:max) / 2
-        let l:obj = a:loclist[l:mid]
-
-        " Binary search to get on the same line
-        if a:loclist[l:mid]['lnum'] < a:line
-            let l:min = l:mid + 1
-        elseif a:loclist[l:mid]['lnum'] > a:line
-            let l:max = l:mid - 1
-        else
-            let l:last_column_match = l:mid
-
-            " Binary search to get the same column, or near it
-            if a:loclist[l:mid]['col'] < a:column
-                let l:min = l:mid + 1
-            elseif a:loclist[l:mid]['col'] > a:column
-                let l:max = l:mid - 1
-            else
-                return l:mid
-            endif
-        endif
-    endwhile
-endfunction
-
 function! ale#cursor#TruncatedEcho(message) abort
     let l:message = a:message
     " Change tabs to spaces.
@@ -87,7 +52,7 @@ function! ale#cursor#EchoCursorWarning(...) abort
 
     let l:pos = getcurpos()
     let l:loclist = g:ale_buffer_loclist_map[l:buffer]
-    let l:index = s:BinarySearch(l:loclist, l:pos[1], l:pos[2])
+    let l:index = ale#util#BinarySearch(l:loclist, l:pos[1], l:pos[2])
 
     if l:index >= 0
         let l:loc = l:loclist[l:index]

autoload/ale/util.vim

@@ -63,3 +63,38 @@ function! ale#util#LocItemCompare(left, right) abort
 
     return 0
 endfunction
+
+" This function will perform a binary search to find a message from the
+" loclist to echo when the cursor moves.
+function! ale#util#BinarySearch(loclist, line, column) abort
+    let l:min = 0
+    let l:max = len(a:loclist) - 1
+    let l:last_column_match = -1
+
+    while 1
+        if l:max < l:min
+            return l:last_column_match
+        endif
+
+        let l:mid = (l:min + l:max) / 2
+        let l:obj = a:loclist[l:mid]
+
+        " Binary search to get on the same line
+        if a:loclist[l:mid]['lnum'] < a:line
+            let l:min = l:mid + 1
+        elseif a:loclist[l:mid]['lnum'] > a:line
+            let l:max = l:mid - 1
+        else
+            let l:last_column_match = l:mid
+
+            " Binary search to get the same column, or near it
+            if a:loclist[l:mid]['col'] < a:column
+                let l:min = l:mid + 1
+            elseif a:loclist[l:mid]['col'] > a:column
+                let l:max = l:mid - 1
+            else
+                return l:mid
+            endif
+        endif
+    endwhile
+endfunction

test/test_loclist_binary_search.vader

@@ -0,0 +1,26 @@
+Before:
+  let g:loclist = [
+  \ {'lnum': 2, 'col': 10},
+  \ {'lnum': 3, 'col': 2},
+  \ {'lnum': 3, 'col': 10},
+  \ {'lnum': 3, 'col': 12},
+  \ {'lnum': 3, 'col': 25},
+  \ {'lnum': 5, 'col': 4},
+  \ {'lnum': 5, 'col': 5},
+  \]
+
+Execute (Exact column matches should be correct):
+   AssertEqual ale#util#BinarySearch(g:loclist, 3, 2), 1
+
+Execute (Off lines, there should be no match):
+   AssertEqual ale#util#BinarySearch(g:loclist, 4, 2), -1
+
+Execute (Near column matches should be taken):
+   AssertEqual ale#util#BinarySearch(g:loclist, 3, 11), 2
+   AssertEqual ale#util#BinarySearch(g:loclist, 3, 13), 4
+
+Execute (Columns before should be taken when the cursor is far ahead):
+   AssertEqual ale#util#BinarySearch(g:loclist, 3, 300), 4
+
+After:
+  unlet g:loclist