cal: implement rfc2445 folding

1 files changed, 44 insertions, 8 deletions

diff --git a/cal.lua b/cal.lua
index 9382524..1c6a453 100755
--- a/cal.lua
+++ b/cal.lua
@@ -8,8 +8,12 @@
 local io            = require "io"
 local ioopen        = io.open
 
+local math          = require "math"
+local mathmin       = math.min
+
 local string        = require "string"
 local stringformat  = string.format
+local stringsub     = string.sub
 
 local table         = require "table"
 local tableconcat   = table.concat
@@ -37,7 +41,9 @@ local noiseln       = mk_out ("stderr", 1, true)
 local debugln       = mk_out ("stderr", 2, true)
 
 local fmt_calendar do
-  local crlf = "\r\n"
+  local fold_rfc2445    = 75  --- B
+  local fold_wsp        = " " --- could be tab as well
+  local crlf            = "\r\n"
 
   local fmt_line = function (ln)
     local params = ""
@@ -62,15 +68,45 @@ local fmt_calendar do
     return stringformat ("%s%s:%s", ln.name, params, ln.value)
   end
 
-  fmt_calendar = function (cal, newline, i, acc)
-    if i == nil then return fmt_calendar (cal, newline, 1, { }) end
-    if i > #cal then return tableconcat (acc, newline or crlf) end
+  local fold_line = function (line, len, newline)
+    local acc = { }
+    local pos = 1
+
+    while pos < #line do
+      local wsp  = ""
+      local pos1
+      if pos == 1 then
+        pos1 = mathmin (#line, pos + len)
+      else
+        pos1 = mathmin (#line, pos + len - 1) --- account for leading whitespace
+        wsp = fold_wsp
+      end
+      acc [#acc + 1] = wsp .. stringsub (line, pos, pos1)
+      pos = pos1
+    end
+
+    return tableconcat (acc, newline)
+  end
+
+  fmt_calendar = function (cal, fold, newline, i, acc)
+    if i == nil then
+      return fmt_calendar (cal,
+                           fold or fold_rfc2445,
+                           newline or crlf,
+                           1, { })
+    end
+    if i > #cal then return tableconcat (acc, newline) end
 
-    local cur = cal [i]
+    local cur  = cal [i]
+    local line = fmt_line (cur)
+
+    if #line > fold then
+      line = fold_line (line, fold, newline)
+    end
 
-    acc [#acc + 1] = fmt_line (cur)
+    acc [#acc + 1] = line
 
-    return fmt_calendar (cal, newline, i + 1, acc)
+    return fmt_calendar (cal, fold, newline, i + 1, acc)
   end
 end
 
@@ -276,7 +312,7 @@ local main = function (argv)
   local raw = loaddata (argv [1])
   local cal = parse_calendar (raw)
 
-  if cal ~= nil then println (fmt_calendar (cal, "\n")) end
+  if cal ~= nil then println (fmt_calendar (cal, nil, "\n")) end
 
   return 0
 end