A function to construct a color by looking it up in the config file

MakeColorSafe() is a gowid function to construct a color based on a
string. Gowid walks through various interpretations of the string to try
to construct the most likely color. This function adds an interpretation
at the beginning, which is as a key in the termshark toml file. This
lets you add something like

[theme]
  flashy = "theme.red"
  red = "#ff0000"

Now you can programmatically use "theme.flashy" as an argument and
construct a color from it.

This function takes an an extra argument, meaning foreground or
background. This allows for pairs of colors to be used:

[theme]
  ocean = ["#ffffff", "#0000ff"]    // white on blue

If you call MakeColorSafe() with "theme.ocean" and theme.Background,
you'll get the blue color.

Values can refer to other keys in the toml file; this can be done up to
10 times before the function gives up (in case there's a loop).

theme/utils.go

@@ -0,0 +1,62 @@
+// Copyright 2019-2020 Graham Clark. All rights reserved.  Use of this source
+// code is governed by the MIT license that can be found in the LICENSE
+// file.
+
+// package theme provides utilities for customizing the styling of termshark.
+package theme
+
+import (
+	"github.com/gcla/gowid"
+	"github.com/gcla/termshark/v2"
+	log "github.com/sirupsen/logrus"
+)
+
+//======================================================================
+
+type Layer int
+
+const (
+	Foreground Layer = 0
+	Background Layer = iota
+)
+
+// MakeColorSafe extends gowid's MakeColorSafe function, prefering to interpret
+// its string argument as a toml file config key lookup first; if this fails, then
+// fall back to gowid.MakeColorSafe, which will then read colors as urwid color names,
+// #-prefixed hex digits, grayscales, etc.
+func MakeColorSafe(s string, l Layer) (gowid.Color, error) {
+	loops := 10
+	cur := s
+	for {
+		next := termshark.ConfString(cur, "")
+		if next != "" {
+			cur = next
+		} else {
+			next := termshark.ConfStringSlice(cur, []string{})
+			if len(next) != 2 {
+				break
+			} else {
+				cur = next[l]
+			}
+		}
+		loops -= 1
+		if loops == 0 {
+			break
+		}
+	}
+	col, err := gowid.MakeColorSafe(cur)
+	if err == nil {
+		return gowid.Color{IColor: col, Id: s}, nil
+	}
+	col, err = gowid.MakeColorSafe(s)
+	if err != nil {
+		log.Infof("Could not understand configured theme color '%s'", s)
+	}
+	return col, err
+}
+
+//======================================================================
+// Local Variables:
+// mode: Go
+// fill-column: 110
+// End: