diff --git a/algorithms/sum_of_digits.typ b/algorithms/sum_of_digits.typ
new file mode 100644
index 0000000..42ff586
--- /dev/null
+++ b/algorithms/sum_of_digits.typ
@@ -0,0 +1,131 @@
+#import "../lib/style.typ": *
+#import "../lib/mapcode.typ": *
+
+== Sum of Digits
+#set math.equation(numbering: none)
+
+Compute the sum of digits of a positive integer $n$. i.e $n in NN, n > 0$
+
+Formal definition:
+$
+"digitsum"(n) = sum_(i=0)^(d-1) floor(n / 10^i) mod 10
+$
+where $d$ is the number of digits in $n$.
+
+Equivalently (recursive):
+$
+"digitsum"(n) = cases(
+  n & "if" n < 10,
+  (n mod 10) + "digitsum"(floor(n / 10)) & "if" n >= 10
+)
+$
+
+Examples:
+- $"digitsum"(123) -> 6$ (since $1 + 2 + 3 = 6$)
+- $"digitsum"(5) -> 5$
+- $"digitsum"(8296) -> 25$ (since $8 + 2 + 9 + 6 = 25$)
+
+*As mapcode:*
+
+_primitives_: 
+- `div`($\/$): integer division (quotient)
+- `mod`($mod$): modulo operation (remainder)
+- `add`($+$): addition
+- `numdigits`: returns number of digits in a number
+
+These operations on $bot$ are undefined (strict).
+
+$ 
+I = NN quad quad quad X_n &= [0..d-1] -> (NN_bot times NN) quad quad quad A = NN
+$
+
+where $d = "numdigits"(n)$
+
+$
+rho(n) & = {i -> (bot, n) | i in [0..d-1]}\
+F(x_n equiv (a_i, r_i))(i) & = cases(
+  (a_i, r_i) & "if " a_i != bot,
+  (r_i mod 10, floor(r_i / 10)) & "if " a_i = bot "and" i = 0,
+  (a_(i-1) + (r_(i-1) mod 10), floor(r_(i-1) / 10)) & "if " a_i = bot "and" i > 0 "and" a_(i-1) != bot,
+  (a_i, floor(r_i / 10)) & "if " a_i = bot "and" i > 0 "and" a_(i-1) = bot
+)\
+pi(x) & = a_(d-1)
+$
+
+where $x_i = (a_i, r_i)$ represents a pair of (accumulated sum, remaining number).
+
+#let inst = 8296;
+#figure(
+  caption: [Sum of digits computation using mapcode for $n = #inst$],
+$
+#{
+  let num_digits = (n) => {
+    if n == 0 { return 1 }
+    let count = 0
+    while n > 0 {
+      count += 1
+      n = calc.quo(n, 10)
+    }
+    count
+  }
+
+  let rho = (inst) => {
+    let d = num_digits(inst)
+    let x = ()
+    for i in range(0, d) {
+      x.push((none, inst))
+    }
+    x
+  }
+
+  let F_i = (x) => ((i,)) => {
+    let curr = x.at(i)
+    if curr.at(0) != none {
+      // Already computed, return as-is
+      curr
+    } else {
+      if i == 0 {
+        (calc.rem(curr.at(1), 10), calc.quo(curr.at(1), 10))
+      } else {
+        let prev = x.at(i - 1)
+        if prev.at(0) != none {
+          (prev.at(0) + calc.rem(prev.at(1), 10), calc.quo(prev.at(1), 10))
+        } else {
+          (curr.at(0), calc.quo(curr.at(1), 10))
+        }
+      }
+    }
+  }
+  let F = map_tensor(F_i, dim: 1)
+
+  let pi = (i) => (x) => x.at(i).at(0)
+
+  let X_h = (x, diff_mask: none) => {
+    let cells = x.enumerate().map(((i, pair)) => {
+      let val = if pair.at(0) != none and pair.at(1) != none {
+        [$(#pair.at(0), #pair.at(1))$]
+      } else if pair.at(0) == none and pair.at(1) != none {
+        [$(bot, #pair.at(1))$]
+      } else {
+        [$(bot, bot)$]
+      }
+      // if diff_mask != none and diff_mask.at(i) {
+      //   rect(fill: yellow.transparentize(70%), inset: 2pt)[$#val$]
+      // } else {
+        rect(stroke: none, inset: 2pt)[$#val$]
+      // }
+    })
+    $vec(delim: "[", ..cells)$
+  }
+
+  let d = num_digits(inst)
+  mapcode-viz(
+    rho, F, pi(d - 1),
+    X_h: X_h,
+    pi_name: [$pi$],
+    group-size: calc.min(7, d),
+    cell-size: 15mm, scale-fig: 85%
+  )(inst)
+}
+$
+)
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diff --git a/main.typ b/main.typ
index 9952d9b..749951e 100644
--- a/main.typ
+++ b/main.typ
@@ -52,3 +52,5 @@ All primitives are _strict_ meaning they do not allow for undefined values (i.e.
 #include "algorithms/LongestCommonSubsequence.typ"
 #pagebreak()
 #include "algorithms/leetcode/P2_add-two-numbers.typ"
+#pagebreak()
+#include "algorithms/sum_of_digits.typ"
\ No newline at end of file