chore: add coloured diff backgrounds

fixes #51

lua/rose-pine/util.lua

@@ -0,0 +1,37 @@
+local util = {}
+
+local function get_byte(value, offset)
+	return bit.band(bit.rshift(value, offset), 0xFF)
+end
+
+local function get_color(color)
+	color = vim.api.nvim_get_color_by_name(color)
+
+	if color == -1 then
+		color = vim.opt.background:get() == 'dark' and 000 or 255255255
+	end
+
+	return { get_byte(color, 16), get_byte(color, 8), get_byte(color, 0) }
+end
+
+---@param fg string foreground color
+---@param bg string background color
+---@param alpha number number between 0 and 1. 0 results in bg, 1 results in fg
+function util.blend(fg, bg, alpha)
+	bg = get_color(bg)
+	fg = get_color(fg)
+
+	local blendChannel = function(i)
+		local ret = (alpha * fg[i] + ((1 - alpha) * bg[i]))
+		return math.floor(math.min(math.max(0, ret), 255) + 0.5)
+	end
+
+	return string.format(
+		'#%02X%02X%02X',
+		blendChannel(1),
+		blendChannel(2),
+		blendChannel(3)
+	)
+end
+
+return util