util.dbuffer: dynamic string buffer

Similar to util.ringbuffer (and shares almost identical API). Differences:

- size limit is optional and dynamic
- does not allocate a fixed buffer of max_size bytes
- focus on simply storing references to existing string objects where possible,
  avoiding unnecessary allocations
- references are still stored in a ring buffer to enable use as a fast FIFO

Optional second parameter to new() provides the number of ring buffer segments. On
Lua 5.2 on my laptop, a segment is ~19 bytes. If the ring buffer fills up, the next
write will compact all strings into a single item.

1 files changed, 95 insertions, 0 deletions

diff --git a/spec/util_dbuffer_spec.lua b/spec/util_dbuffer_spec.lua
new file mode 100644
index 00000000..854f3125
--- /dev/null
+++ b/spec/util_dbuffer_spec.lua
@@ -0,0 +1,95 @@
+local dbuffer = require "util.dbuffer";
+describe("util.dbuffer", function ()
+	describe("#new", function ()
+		it("has a constructor", function ()
+			assert.Function(dbuffer.new);
+		end);
+		it("can be created", function ()
+			assert.truthy(dbuffer.new());
+		end);
+		it("won't create an empty buffer", function ()
+			assert.falsy(dbuffer.new(0));
+		end);
+		it("won't create a negatively sized buffer", function ()
+			assert.falsy(dbuffer.new(-1));
+		end);
+	end);
+	describe(":write", function ()
+		local b = dbuffer.new();
+		it("works", function ()
+			assert.truthy(b:write("hi"));
+		end);
+	end);
+
+	describe(":discard", function ()
+		local b = dbuffer.new();
+		it("works", function ()
+			assert.truthy(b:write("hello world"));
+			assert.truthy(b:discard(6));
+			assert.equal(5, #b);
+			assert.equal("world", b:read(5));
+		end);
+	end);
+
+	describe(":sub", function ()
+		-- Helper function to compare buffer:sub() with string:sub()
+		local s = "hello world";
+		local function test_sub(b, x, y)
+			local string_result, buffer_result = s:sub(x, y), b:sub(x, y);
+			assert.equals(string_result, buffer_result, ("buffer:sub(%d, %s) does not match string:sub()"):format(x, y and ("%d"):format(y) or "nil"));
+		end
+
+		it("works", function ()
+			local b = dbuffer.new();
+			assert.truthy(b:write("hello world"));
+			assert.equals("hello", b:sub(1, 5));
+		end);
+
+		it("supports optional end parameter", function ()
+			local b = dbuffer.new();
+			assert.truthy(b:write("hello world"));
+			assert.equals("hello world", b:sub(1));
+			assert.equals("world", b:sub(-5));
+		end);
+
+		it("is equivalent to string:sub", function ()
+			local b = dbuffer.new(11);
+			assert.truthy(b:write(s));
+			for i = -13, 13 do
+				for j = -13, 13 do
+					test_sub(b, i, j);
+				end
+			end
+		end);
+	end);
+
+	describe(":byte", function ()
+		-- Helper function to compare buffer:byte() with string:byte()
+		local s = "hello world"
+		local function test_byte(b, x, y)
+			local string_result, buffer_result = {s:byte(x, y)}, {b:byte(x, y)};
+			assert.same(string_result, buffer_result, ("buffer:byte(%d, %s) does not match string:byte()"):format(x, y and ("%d"):format(y) or "nil"));
+		end
+
+		it("is equivalent to string:byte", function ()
+			local b = dbuffer.new(11);
+			assert.truthy(b:write(s));
+			test_byte(b, 1);
+			test_byte(b, 3);
+			test_byte(b, -1);
+			test_byte(b, -3);
+			for i = -13, 13 do
+				for j = -13, 13 do
+					test_byte(b, i, j);
+				end
+			end
+		end);
+
+		it("works with characters > 127", function ()
+			local b = dbuffer.new();
+			b:write(string.char(0, 140));
+			local r = { b:byte(1, 2) };
+			assert.same({ 0, 140 }, r);
+		end);
+	end);
+end);