From 10123fd76ac2c071eaebce066046354324a31931 Mon Sep 17 00:00:00 2001
From: Ayush Rawal <royalrawal.2001@gmail.com>
Date: Sun, 16 Nov 2025 23:57:38 +0530
Subject: [PATCH] add coin change

---
 algorithms/CoinChange.typ | 131 ++++++++++++++++++++++++++++++++++++++
 main.typ                  |   2 +
 2 files changed, 133 insertions(+)
 create mode 100644 algorithms/CoinChange.typ

diff --git a/algorithms/CoinChange.typ b/algorithms/CoinChange.typ
new file mode 100644
index 0000000..39afeab
--- /dev/null
+++ b/algorithms/CoinChange.typ
@@ -0,0 +1,131 @@
+#import "../lib/style.typ": *
+#import "../lib/mapcode.typ": *
+
+== Coin Change (dynamic programming, mapcode)
+
+Compute the minimum number of coins needed to make a given amount using a given set of coin denominations. Each coin can be used multiple times.
+
+Formal definition:
+$
+"MinCoins"(a) = cases(
+  0 & "if " a = 0,
+  1 + min_(c in "coins", c <= a) "MinCoins"(a - c) & "if " a > 0
+)
+$
+
+*As mapcode:*
+
+_primitives_: `min`, `add`($+$), `leq`($<=$)
+
+$
+I = a:NN times "coins":[NN]^k quad quad quad
+X_a & = [0..a] -> NN union {infinity}_bot quad quad quad
+A = NN union {infinity}\
+rho(a, "coins") & = { i -> bot | i in {0 dots a}} \
+F_("coins")(x_i) & = cases(
+    0 & "if " i = 0,
+    1 + min_(c in "coins", c <= i) x_(i - c) & "if " i > 0 "and all deps met",
+    infinity & "if " i > 0 "and no coin works"
+  )\
+pi_a (x) & = x_a
+$
+
+#let inst_amount = 13;
+#let inst_coins = (1, 4, 6);
+
+#figure(
+  caption: [Coin Change DP array for amount = #inst_amount, coins = #inst_coins.],
+$#{
+  let rho = ((inst_amount, inst_coins)) => {
+    let x = ()
+    for i in range(0, inst_amount + 1) {
+      x.push(none)
+    }
+    x
+  }
+
+  let F_i = ((coins,)) => (x) => ((i,)) => {
+    if i == 0 {
+      0
+    } else {
+      // Check if all dependencies are met
+      let all_deps_met = true
+      for coin in coins {
+        if i >= coin and x.at(i - coin) == none {
+          all_deps_met = false
+        }
+      }
+
+      if all_deps_met {
+        // Compute minimum
+        let min_val = calc.inf
+        for coin in coins {
+          if i >= coin {
+            let dep_val = x.at(i - coin)
+            if dep_val != calc.inf {
+              min_val = calc.min(min_val, 1 + dep_val)
+            }
+          }
+        }
+        min_val
+      } else {
+        none
+      }
+    }
+  }
+
+  let F = ((coins,)) => map_tensor(F_i((coins,)), dim: 1)
+
+  let pi = ((amount,)) => (x) => x.at(amount)
+
+  let I_h((inst_amount, inst_coins)) = {
+    [
+      $a: #inst_amount$,
+      $"coins": vec(..#inst_coins.map(c => [#c]), delim: "[")_(#inst_coins.len())$,
+    ]
+  }
+
+  // visualization: 1D array as vertical table (index in left column, value in right column)
+  let x_h(x, diff_mask:none) = {
+    set text(weight: "bold")
+    let rows = ()
+
+    // Each row contains: [index, value]
+    for i in range(0, x.len()) {
+      let row = ()
+      
+      // Column 1: index (orange header)
+      row.push(rect(fill: orange.transparentize(70%), inset: 4pt)[$#i$])
+      
+      // Column 2: value
+      let val = if x.at(i) == none {
+        [$bot$]
+      } else if x.at(i) == calc.inf {
+        [$infinity$]
+      } else {
+        [$#x.at(i)$]
+      }
+      
+      if diff_mask != none and diff_mask.at(i) {
+        row.push(rect(stroke: gray, fill: yellow.transparentize(70%), inset: 4pt)[$#val$])
+      } else {
+        row.push(rect(stroke: gray, inset: 4pt)[$#val$])
+      }
+      
+      rows.push(grid(columns: 2 * (20pt,), rows: 16pt, align: center + horizon, ..row))
+    }
+
+    grid(align: center, ..rows)
+  }
+
+  mapcode-viz(
+    rho, F((inst_coins,)), pi((inst_amount,)),
+    I_h: I_h,
+    X_h: x_h,
+    F_name: [$F_("coins")$],
+    pi_name: [$\pi_(#inst_amount)$],
+    group-size: 5,
+    cell-size: 30mm, scale-fig: 75%
+  )((inst_amount, inst_coins))
+}$)
+
diff --git a/main.typ b/main.typ
index 9952d9b..3058b0d 100644
--- a/main.typ
+++ b/main.typ
@@ -51,4 +51,6 @@ All primitives are _strict_ meaning they do not allow for undefined values (i.e.
 #pagebreak()
 #include "algorithms/LongestCommonSubsequence.typ"
 #pagebreak()
+#include "algorithms/CoinChange.typ"
+#pagebreak()
 #include "algorithms/leetcode/P2_add-two-numbers.typ"