refactor(indent): refactor computing of a string's indent size (#27252)

The `get_indent_str_vtab()` function currently calls `tabstop_padding()`
every time a tab is encountered (unless tabstops aren't used).
`tabstop_padding()` either does a division by 'tabstop' If 'vartabstop'
is not set, or iterates through the 'vartabstop' list to find current
tab width. 

Since the virtual column only increases, we can keep track of where the
next tabstop would be, and update this information once it was reached.

`get_indent_str_vtab()` also depends on 'listchars' "tab" value from the
current window, even though it may be called for a line from the same
buffer in a different window. In most cases, it is called with tabstops
enabled (last argument was `false`), so I split the function into one
that uses tabstops and the other that doesn't.

I removed `get_indent_str()` since I couldn't find any calls to it.

1 files changed, 30 insertions, 0 deletions

diff --git a/test/unit/indent_spec.lua b/test/unit/indent_spec.lua
index f93f6d7581..7902918c54 100644
--- a/test/unit/indent_spec.lua
+++ b/test/unit/indent_spec.lua
@@ -1,6 +1,8 @@
 local helpers = require('test.unit.helpers')(after_each)
 local itp = helpers.gen_itp(it)
 
+local to_cstr = helpers.to_cstr
+local ffi = helpers.ffi
 local eq = helpers.eq
 
 local indent = helpers.cimport('./src/nvim/indent.h')
@@ -28,3 +30,31 @@ describe('get_sts_value', function()
     eq(tabstop, indent.get_sts_value())
   end)
 end)
+
+describe('indent_size_ts()', function()
+  itp('works for spaces', function()
+    local line = to_cstr((' '):rep(7) .. 'a ')
+    eq(7, indent.indent_size_ts(line, 100, nil))
+  end)
+
+  itp('works for tabs and spaces', function()
+    local line = to_cstr('   \t  \t \t\t   a ')
+    eq(19, indent.indent_size_ts(line, 4, nil))
+  end)
+
+  itp('works for tabs and spaces with empty vts', function()
+    local vts = ffi.new('int[1]') -- zero initialized => first element (size) == 0
+    local line = to_cstr('   \t  \t \t\t       a ')
+    eq(23, indent.indent_size_ts(line, 4, vts))
+  end)
+
+  itp('works for tabs and spaces with vts', function()
+    local vts = ffi.new('int[3]')
+    vts[0] = 2 -- zero indexed
+    vts[1] = 7
+    vts[2] = 2
+
+    local line = to_cstr('      \t  \t \t\t   a ')
+    eq(18, indent.indent_size_ts(line, 4, vts))
+  end)
+end)