From 41753b358251eb737709bbc4172cb551ef41ef87 Mon Sep 17 00:00:00 2001
From: Matthijs Kooijman <matthijs@stdin.nl>
Date: Fri, 28 Aug 2009 15:23:46 +0200
Subject: [PATCH] Make pret-lam preserve alignment using whitespace.

This is slightly complicated, but it should work properly in most cases.
---
 Utils/Formats.tex |  4 +++
 pret-lam.lua      | 71 ++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 71 insertions(+), 4 deletions(-)

diff --git a/Utils/Formats.tex b/Utils/Formats.tex
index 993a988..aae99ed 100644
--- a/Utils/Formats.tex
+++ b/Utils/Formats.tex
@@ -25,3 +25,7 @@
    background=,
    frame=off]
 
+\def\setwidthof#1\to#2%
+{\bgroup
+\setbox\scratchbox\hbox{#1}%
+\expanded{\egroup\def\noexpand#2{\the\wd\scratchbox}}}
diff --git a/pret-lam.lua b/pret-lam.lua
index 54acfd1..98244b0 100644
--- a/pret-lam.lua
+++ b/pret-lam.lua
@@ -35,6 +35,12 @@ local keywords = {
     ['in'] = {},
 }
 
+local in_block = 0
+local submatches = {}
+local bases = {}
+-- Store the last line for each indent level
+local indentlines = {}
+
 -- See if str starts with a symbol, and return the remaining string and that
 -- symbol. If no symbol from the table is matched, just returns the first
 -- character.  We can do a lookup directly, since symbols can be different in
@@ -101,9 +107,63 @@ local function do_subscripts(word)
     return word
 end
 
-local in_block = 0
-local submatches = {}
-local bases = {}
+-- Do proper aligning for subsequent lines. For example, in 
+--   foo = bar
+--       | baz
+-- We replace the spaces in the second line with a skip with the same with as
+-- "foo ", to align the | with the =.
+-- For this, we keep a table "indentlines", which contains all previous lines
+-- with smaller indent levels that are still "in scope" (e.g., have not yet
+-- been followed by a line with a smaller indent level). For example:
+--   line1
+--     line2
+--       line3
+--     line4
+--       line5
+-- After the last line, the table will contain:
+--   { 0 = "line1", 2 = "  line4", 4 = "    line5"}
+--   In other words, line3 is no longer in scope since it is "hidden" by
+--   line4, and line is no longer in scope since it is replaced by line4.
+local function do_indent(line)
+    newind, rest = utf.match(line, '^(%s*)(.*)')
+    prev = -1
+    -- Loop all the previous lines
+    for indent, unused in pairs(indentlines) do
+        if indent > #newind then
+            -- Remove any lines with a larger indent
+            indentlines[indent] = nil
+        elseif indent < #newind and indent > prev then
+            -- Find the last line (e.g, with the highest indent) with an
+            -- indent smaller than the new indent. This is the line from which
+            -- we need to copy the indent.
+            prev = indent
+        end
+    end
+    
+    -- Always store this line, possibly overwriting a previous line with the
+    -- same indent
+    indentlines[#newind] = line
+
+    if prev ~= -1 then
+        -- If there is a previous line with a smaller indent, make sure we
+        -- align with it. We do this by taking a prefix from that previous
+        -- line just as long as our indent. This gives us a bunch of
+        -- whitespace, with a few non-whitespace characters. We find out the
+        -- width of this prefix, and put whitespace just as wide as that
+        -- prefix before the current line, instead of the whitespace
+        -- characters that were there.
+        -- Doing this is slightly risky, since the prefix might contain
+        -- unfinished markup (e.g., \foo{bar without the closing }). We might
+        -- need to solve this later.
+        copyind = utf.sub(indentlines[prev], 1, #newind)
+        setwidth = "\\setwidthof{" .. copyind .. "}\\to\\pretlamalignwidth"
+        hskip = "\\hskip\\pretlamalignwidth"
+        return "{" .. setwidth .. hskip .. "}" .. rest
+    end
+        -- No previous line? Just return the unmodified line then
+        return line
+end
+
 
 -- Mark the begin of a block of lambda formatted buffers or expressions. This
 -- means that, until you call end_of_block again, the subscript bases are
@@ -135,7 +195,9 @@ function vis.begin_of_display()
         -- above.
         bases = {}
     end
+    indentlines = {}
 end
+    
 
 -- Make things work for inline typeing (e.g., \type{}) as well.
 vis.begin_of_inline = vis.begin_of_display
@@ -144,6 +206,7 @@ vis.end_of_inline = vis.end_of_display
 function vis.flush_line(str,nested)
     local result, state = { }, 0
     local finish, change = buffers.finish_state, buffers.change_state
+    str = do_indent(str)
     -- Set the colorscheme, which is used by finish_state and change_state
     buffers.currentcolors = colors
     while str ~= "" do
@@ -167,7 +230,7 @@ function vis.flush_line(str,nested)
         -- Append the resulting token
         result[#result+1] = word or symbol
     end
-
+    
     state = finish(state, result)
     buffers.flush_result(result,nested)
 end
-- 
2.30.2