From f0f5b15fbe827572e335ca825d627f05325ee6e9 Mon Sep 17 00:00:00 2001
From: heav <hheav3@gmail.com>
Date: Thu, 27 Oct 2022 01:31:54 +0000
Subject: added filter, folds and sum/product to table library

---
 czzc/table.lua | 77 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 76 insertions(+), 1 deletion(-)

diff --git a/czzc/table.lua b/czzc/table.lua
index bb34f2f..92619eb 100644
--- a/czzc/table.lua
+++ b/czzc/table.lua
@@ -23,11 +23,86 @@ end
 
 -- replaces all values of a table with fn(value).
 M.map = function(fn, tbl)
-    local res = M.copy(tbl)
+    local res = {}
     for k, v in pairs(tbl) do
         res[k] = fn(v)
     end
     return res
 end
 
+-- in-place version of the above.
+M.map_ip = function(fn, tbl)
+    for k, v in pairs(tbl) do
+        tbl[k] = fn(v)
+    end
+    return tbl -- redundant, technically, but i thought i'd include it anyway.
+end
+
+-- discards or keeps values from a table based on a predicate.
+-- this one only supports arrays, not maps.
+M.filter = function(pred, arr)
+    local res = {}
+    for i, v in ipairs(arr) do
+        if pred(v) then
+            table.insert(res, v)
+        end
+    end
+    return res
+end
+
+M.filter_ip = function(pred, arr) -- in-place.
+    for i = #arr, 1, -1 do -- we need to iterate backwards here to prevent the.
+        if not pred(arr[i]) then
+            table.remove(arr, i)
+        end
+    end
+    return arr
+end
+
+-- like filter, but works on map-style tables instead.
+M.filter_map = function(pred, map)
+    local res = {}
+    for k, v in pairs(map) do
+        if pred(v) then
+            res[k] = v
+        end
+    end
+    return res
+end
+
+M.filter_map_ip = function(pred, map) -- in-place.
+    for k, v in pairs(map) do
+        if not pred(v) then
+            map[k] = nil
+        end
+    end
+    return map
+end
+
+-- "folds" an operation over an array. i.e foldr(*, e, {a, b, c, d}) = a * (b * (c * (d * e))).
+M.foldr = function(op, startval, arr) 
+    local res = startval or 0 -- sensible default?
+    for i = #arr, 1, -1 do
+        res = op(res, arr[i])
+    end
+    return res
+end
+
+-- same as above, but the other way i.e ((e * a) * b) * c
+M.foldl = function(op, startval, arr)
+    local res = startval or 0 
+    for i=1, #arr do
+        res = op(res, arr[i])
+    end
+    return res
+end
+
+M.sum = function(arr)
+    return M.foldl(function(a,b) return a+b end, 0, arr)
+end
+
+M.product = function(arr)
+    return M.foldl(function(a,b) return a*b end, 0, arr)
+end
+
 return M
-- 
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