move application-aware utility functions to separate module

1 files changed, 34 insertions, 0 deletions

diff --git a/util.lua b/util.lua
index 99f8fe8..04e1bf6 100644
--- a/util.lua
+++ b/util.lua
@@ -91,4 +91,38 @@ function util.write_json_file(path, data)
 	f:close()
 end
 
+-- merges tables `t' and `u', using key `k' for identity. the order from `u' is
+-- always respected, preserving the relative order from `t' when possible.
+function util.stable_kmerge(t, u, k)
+	local pos = {}
+	for i, v in ipairs(u) do
+		pos[v[k]] = i
+	end
+
+	local res = {}
+	local seen = {}
+	local function append(v)
+		if not seen[v[k]] then
+			res[#res+1] = v
+			seen[v[k]] = true
+		end
+	end
+
+	local ind = 1
+	for _, v in ipairs(t) do
+		if pos[v[k]] then
+			while ind <= pos[v[k]] do
+				append(u[ind])
+				ind = ind + 1
+			end
+		end
+		append(v)
+	end
+	for i = ind, #u do
+		append(u[i])
+	end
+
+	return res
+end
+
 return util