CMakeLists.txt

@@ -11,6 +11,39 @@ project(
 
 find_package(GTest CONFIG REQUIRED)
 
+file(GLOB LIB_SRCS "common/**.cpp")
+
+add_library(
+  common STATIC
+    ${LIB_SRCS}
+)
+
+target_include_directories(
+  common PUBLIC
+    common/
+)
+
+file(GLOB LIB_TESTS "common-tests/src/**.cpp")
+
+foreach(FILE IN LISTS LIB_TESTS)
+  cmake_path(GET FILE STEM TEST)
+
+  add_executable("common-tests-${TEST}" "${FILE}")
+
+  target_link_libraries(
+    "common-tests-${TEST}" PRIVATE
+        common
+        GTest::gtest_main
+  )
+
+  set_target_properties(
+    "common-tests-${TEST}" PROPERTIES
+        RUNTIME_OUTPUT_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/common-tests/bin"
+        RUNTIME_OUTPUT_NAME "test-${TEST}"
+        SUFFIX ""
+  )
+endforeach()
+
 file(GLOB SOURCES "problem-*/")
 
 foreach(FILE IN LISTS SOURCES)
@@ -37,24 +70,24 @@ foreach(FILE IN LISTS SOURCES)
     
     target_link_libraries(
         "${PROBLEM}" PRIVATE
+          common
     )
 
     target_link_libraries(
         "test-${PROBLEM}" PRIVATE
+            common
             GTest::gtest_main
     )
 
     target_include_directories(
         "${PROBLEM}" PRIVATE
             "${FILE}/src"
-            "${CMAKE_CURRENT_SOURCE_DIR}/common"
     )
 
 
     target_include_directories(
         "test-${PROBLEM}" PRIVATE
             "${FILE}/src"
-            "${CMAKE_CURRENT_SOURCE_DIR}/common"
     )
 
     set_target_properties(

common-tests/src/bigint.cpp

@@ -0,0 +1,229 @@
+/*
+ * Project Euler Solutions - Test File
+ * ------------------------------------
+ * Problem     : Testing
+ * URL         : https://projecteuler.net/problem=test
+ *
+ * Author      : erick-alcachofa
+ * Created     : Friday, September 05 2025
+ *
+ * Notes:
+ * -
+ *
+ * License     : GNU Affero General Public License v3.0 (AGPLv3)
+ *               https://www.gnu.org/licenses/agpl-3.0.html
+ */
+
+#include <gtest/gtest.h>
+
+#include "bigint.hpp"
+
+// Helper macro for creating BigInt from string, fails test on error
+#define BI(str) BigInt::fromString(str).value()
+
+TEST(FromString, HandlesPositiveNumbers) {
+  EXPECT_EQ(BI("12345").toString(), "12345");
+  EXPECT_EQ(BI("0").toString(), "0");
+  EXPECT_EQ(BI("99").toString(), "99");
+  EXPECT_EQ(BI("100").toString(), "100");
+}
+
+TEST(FromString, HandlesNegativeNumbers) {
+  EXPECT_EQ(BI("-12345").toString(), "-12345");
+  EXPECT_EQ(BI("-99").toString(), "-99");
+}
+
+TEST(FromString, HandlesZero) {
+  EXPECT_EQ(BI("0").toString(), "0");
+  EXPECT_EQ(BI("-0").toString(), "0"); // Canonical zero
+}
+
+TEST(FromString, HandlesWhitespaceAndSigns) {
+  EXPECT_EQ(BI("  123  ").toString(), "123");
+  EXPECT_EQ(BI("+456").toString(), "456");
+  EXPECT_EQ(BI("  -789 ").toString(), "-789");
+}
+
+TEST(FromString, HandlesInvalidInput) {
+  EXPECT_FALSE(BigInt::fromString("").has_value());
+  EXPECT_FALSE(BigInt::fromString("-").has_value());
+  EXPECT_FALSE(BigInt::fromString("12a34").has_value());
+  EXPECT_FALSE(BigInt::fromString("abc").has_value());
+}
+
+TEST(Addition, PositivePlusPositive) {
+  EXPECT_EQ((BI("123") + BI("456")).toString(), "579");
+  EXPECT_EQ((BI("99") + BI("1")).toString(), "100"); // With carry
+  EXPECT_EQ((BI("9999") + BI("1")).toString(), "10000");
+}
+
+TEST(Addition, NegativePlusNegative) {
+  EXPECT_EQ((BI("-123") + BI("-456")).toString(), "-579");
+  EXPECT_EQ((BI("-99") + BI("-1")).toString(), "-100");
+}
+
+TEST(Addition, MixedSigns) {
+  EXPECT_EQ((BI("500") + BI("-200")).toString(), "300");
+  EXPECT_EQ((BI("200") + BI("-500")).toString(), "-300");
+  EXPECT_EQ((BI("-500") + BI("200")).toString(), "-300");
+  EXPECT_EQ((BI("-200") + BI("500")).toString(), "300");
+  EXPECT_EQ((BI("200") + BI("-200")).toString(), "0");
+}
+
+TEST(Subtraction, PositiveMinusPositive) {
+  EXPECT_EQ((BI("500") - BI("200")).toString(), "300");
+  EXPECT_EQ((BI("200") - BI("500")).toString(), "-300");
+  EXPECT_EQ((BI("100") - BI("1")).toString(), "99"); // With borrow
+}
+
+TEST(Subtraction, MixedSigns) {
+  EXPECT_EQ((BI("200") - BI("-300")).toString(), "500");
+  EXPECT_EQ((BI("-200") - BI("300")).toString(), "-500");
+  EXPECT_EQ((BI("-500") - BI("-200")).toString(), "-300");
+}
+
+TEST(Subtraction, ResultingInZero) {
+  EXPECT_EQ((BI("200") - BI("200")).toString(), "0");
+  EXPECT_EQ((BI("-200") - BI("-200")).toString(), "0");
+}
+
+TEST(Multiplication, BasicCases) {
+  EXPECT_EQ((BI("12") * BI("10")).toString(), "120");
+  EXPECT_EQ((BI("123") * BI("456")).toString(), "56088");
+  EXPECT_EQ((BI("99") * BI("99")).toString(), "9801");
+}
+
+TEST(Multiplication, WithZero) {
+  EXPECT_EQ((BI("123") * BI("0")).toString(), "0");
+  EXPECT_EQ((BI("0") * BI("123")).toString(), "0");
+  EXPECT_EQ((BI("-123") * BI("0")).toString(), "0");
+}
+
+TEST(Multiplication, WithNegativeNumbers) {
+  EXPECT_EQ((BI("-12") * BI("10")).toString(), "-120");
+  EXPECT_EQ((BI("12") * BI("-10")).toString(), "-120");
+  EXPECT_EQ((BI("-12") * BI("-10")).toString(), "120");
+}
+
+TEST(DivisionAndRemainder, DivRemPositive) {
+    auto result = BigInt::divmod(BI("12345"), BI("54"));
+    ASSERT_TRUE(result.has_value());
+    EXPECT_EQ(result->quotient.toString(), "228");
+    EXPECT_EQ(result->remainder.toString(), "33");
+
+    result = BigInt::divmod(BI("100"), BI("10"));
+    ASSERT_TRUE(result.has_value());
+    EXPECT_EQ(result->quotient.toString(), "10");
+    EXPECT_EQ(result->remainder.toString(), "0");
+}
+
+TEST(DivisionAndRemainder, DivRemNegative) {
+    // -12345 / 54 -> Q = -228, R = -33
+    auto result = BigInt::divmod(BI("-12345"), BI("54"));
+    ASSERT_TRUE(result.has_value());
+    EXPECT_EQ(result->quotient.toString(), "-228");
+    EXPECT_EQ(result->remainder.toString(), "-33");
+
+    // 12345 / -54 -> Q = -228, R = 33
+    result = BigInt::divmod(BI("12345"), BI("-54"));
+    ASSERT_TRUE(result.has_value());
+    EXPECT_EQ(result->quotient.toString(), "-228");
+    EXPECT_EQ(result->remainder.toString(), "33");
+
+    // -12345 / -54 -> Q = 228, R = -33
+    result = BigInt::divmod(BI("-12345"), BI("-54"));
+    ASSERT_TRUE(result.has_value());
+    EXPECT_EQ(result->quotient.toString(), "228");
+    EXPECT_EQ(result->remainder.toString(), "-33");
+}
+
+TEST(DivisionAndRemainder, QuotientOnly) {
+    auto q = BigInt::quotient(BI("56088"), BI("123"));
+    ASSERT_TRUE(q.has_value());
+    EXPECT_EQ(q->toString(), "456");
+
+    q = BigInt::quotient(BI("-100"), BI("9"));
+    ASSERT_TRUE(q.has_value());
+    EXPECT_EQ(q->toString(), "-11");
+}
+
+TEST(DivisionAndRemainder, RemainderOnly) {
+    auto r_pos = BigInt::remainder(BI("100"), BI("9"));
+    ASSERT_TRUE(r_pos.has_value());
+    EXPECT_EQ(r_pos->toString(), "1");
+
+    auto r_neg = BigInt::remainder(BI("-100"), BI("9"));
+    ASSERT_TRUE(r_neg.has_value());
+    EXPECT_EQ(r_neg->toString(), "-1");
+}
+
+TEST(DivisionAndRemainder, DivisionByZero) {
+    EXPECT_FALSE(BigInt::divmod(BI("100"), BI("0")).has_value());
+    EXPECT_FALSE(BigInt::quotient(BI("100"), BI("0")).has_value());
+    EXPECT_FALSE(BigInt::remainder(BI("100"), BI("0")).has_value());
+}
+
+TEST(DivisionAndRemainder, DividendSmallerThanDivisor) {
+    auto result = BigInt::divmod(BI("10"), BI("100"));
+    ASSERT_TRUE(result.has_value());
+    EXPECT_EQ(result->quotient.toString(), "0");
+    EXPECT_EQ(result->remainder.toString(), "10");
+
+    result = BigInt::divmod(BI("-10"), BI("100"));
+    ASSERT_TRUE(result.has_value());
+    EXPECT_EQ(result->quotient.toString(), "0");
+    EXPECT_EQ(result->remainder.toString(), "-10");
+}
+
+TEST(Comparison, AllOperators) {
+  EXPECT_TRUE(BI("100") > BI("99"));
+  EXPECT_TRUE(BI("99") < BI("100"));
+  EXPECT_TRUE(BI("-99") > BI("-100"));
+  EXPECT_TRUE(BI("-100") < BI("-99"));
+  EXPECT_TRUE(BI("100") >= BI("100"));
+  EXPECT_TRUE(BI("100") <= BI("100"));
+  EXPECT_TRUE(BI("100") == BI("100"));
+  EXPECT_TRUE(BI("100") != BI("101"));
+  EXPECT_TRUE(BI("100") > BI("-100"));
+  EXPECT_TRUE(BI("-100") < BI("100"));
+}
+
+TEST(LargeValues, HandlesValuesExceedingULLONG_MAX) {
+  // ULLONG_MAX is 18,446,744,073,709,551,615. These numbers are larger.
+  const auto big_num1 = BI("20000000000000000000"); // 2 * 10^19
+  const auto big_num2 = BI("10000000000000000000"); // 1 * 10^19
+  const auto neg_big_num1 = BI("-20000000000000000000");
+
+  // Addition
+  EXPECT_EQ((big_num1 + big_num2).toString(), "30000000000000000000");
+
+  // Subtraction
+  EXPECT_EQ((big_num1 - big_num2).toString(), "10000000000000000000");
+
+  // Multiplication
+  EXPECT_EQ((big_num1 * big_num2).toString(),
+            "200000000000000000000000000000000000000");
+
+  // Division and Remainder
+  auto result = BigInt::divmod(big_num1, big_num2);
+  ASSERT_TRUE(result.has_value());
+  EXPECT_EQ(result->quotient.toString(), "2");
+  EXPECT_EQ(result->remainder.toString(), "0");
+
+  // Comparisons
+  EXPECT_TRUE(big_num1 > big_num2);
+  EXPECT_TRUE(big_num2 < big_num1);
+  EXPECT_TRUE(big_num1 > neg_big_num1);
+  EXPECT_TRUE(neg_big_num1 < big_num2);
+
+  EXPECT_EQ((BI("7284792756374829238474859") * BI("28347299847928374918237"))
+                .toString(),
+            "206504204594973924026042468998840507198705103583");
+
+  result = BigInt::divmod(
+      BI("206504204594973924026042468998840507198705103583"), BI("38294791"));
+  ASSERT_TRUE(result.has_value());
+  EXPECT_EQ(result->quotient.toString(),
+            "5392488095704032541293735458664352214344");
+  EXPECT_EQ(result->remainder.toString(), "14421479");
+}

common/bigint.cpp

@@ -0,0 +1,420 @@
+#include "bigint.hpp"
+
+#include <algorithm>
+#include <ranges>
+#include <format>
+#include <print>
+#include <sstream>
+
+std::expected<BigInt, std::string>
+BigInt::fromString(std::string_view value) {
+  BigInt result;
+
+  value = trim(value, isSpace);
+
+  if (value.empty()) {
+    return std::unexpected{"Invalid value: input string is empty."};
+  }
+
+  if (value.front() == '-') {
+    result.isNegative = true;
+    value.remove_prefix(1);
+  } else if (value.front() == '+') {
+    value.remove_prefix(1);
+  }
+
+  if (value.empty()) {
+    return std::unexpected{"Invalid value: string contains only a sign."};
+  }
+
+  if (rg::any_of(value, [](u8 ch) { return !std::isdigit(ch); })) {
+    return std::unexpected{"Invalid value: contains non-digit characters."};
+  }
+
+  result.digits.reserve(value.size() / 2 + 1);
+
+  auto it = value.begin();
+
+  if ((value.size() % 2) == 1) {
+    result.digits.push_back(u8(*it++ - '0'));
+  }
+
+  while (it != value.end()) {
+    u8 cval =  u8((*it++ - '0') * 10);
+       cval += u8((*it++ - '0'));
+    result.digits.push_back(cval);
+  }
+
+  rg::reverse(result.digits);
+
+  result.trimLeadingZeroes();
+
+  if (result.isZero()) {
+    result.isNegative = false;
+  }
+
+  return result;
+}
+
+std::string BigInt::toString() const {
+  if (this->isZero()) {
+    return "0";
+  }
+
+  std::stringstream ss;
+
+  if (isNegative) {
+    ss << "-";
+  }
+
+  ss << i32(digits.back());
+
+  for (const auto &d : this->digits | vw::reverse | vw::drop(1)) {
+    ss << std::format("{:02}", i32(d));
+  }
+
+  return ss.str();
+}
+
+void BigInt::trimLeadingZeroes() noexcept {
+  while (!digits.empty() && digits.back() == 0) {
+    digits.pop_back();
+  }
+}
+
+BigInt BigInt::addMagnitudes(const BigInt &lhs, const BigInt &rhs) {
+  BigInt result;
+  u16 carry = 0;
+
+  const auto &longer = [&] -> const BigInt & {
+    if (lhs.size() > rhs.size()) {
+      return lhs;
+    }
+    return rhs;
+  }();
+
+  const auto &shorter = [&] -> const BigInt & {
+    if (lhs.size() <= rhs.size()) {
+      return lhs;
+    }
+    return rhs;
+  }();
+
+  result.digits.reserve(longer.size() + 1);
+
+  for (u64 i = 0; i < shorter.size(); ++i) {
+    u16 sum = shorter.digits[i] + longer.digits[i] + carry;
+
+    carry = sum / Base;
+    sum %= Base;
+
+    result.digits.push_back(sum);
+  }
+
+  for (u64 i = shorter.size(); i < longer.size(); ++i) {
+    u16 sum = longer.digits[i] + carry;
+
+    carry = sum / Base;
+    sum %= Base;
+
+    result.digits.push_back(sum);
+  }
+
+  if (carry != 0) {
+    result.digits.push_back(carry);
+  }
+
+  return result;
+}
+
+BigInt BigInt::subMagnitudes(const BigInt &lhs, const BigInt &rhs) {
+  BigInt result;
+  i16 borrow = 0;
+
+  result.digits.reserve(lhs.size() + 1);
+
+  for (u64 i = 0; i < rhs.size(); ++i) {
+    i16 diff = lhs.digits[i] - rhs.digits[i] - borrow;
+
+    if (diff < 0) {
+      diff += Base;
+      borrow = 1;
+    } else {
+      borrow = 0;
+    }
+
+    result.digits.push_back(u8(diff));
+  }
+
+  for (u64 i = rhs.size(); i < lhs.size(); ++i) {
+    i16 diff = lhs.digits[i] - borrow;
+
+    if (diff < 0) {
+      diff += Base;
+      borrow = 1;
+    } else {
+      borrow = 0;
+    }
+
+    result.digits.push_back(u8(diff));
+  }
+
+  result.trimLeadingZeroes();
+
+  return result;
+}
+
+BigInt BigInt::multiplyMagnitudes(const BigInt &lhs, const BigInt &rhs) {
+  if (lhs.isZero() || rhs.isZero()) {
+    return BigInt{};
+  }
+
+  std::vector<u32> accumulator(lhs.size() + rhs.size() + 1, 0);
+
+  const auto &longer = [&] -> const BigInt & {
+    if (lhs.size() > rhs.size()) {
+      return lhs;
+    }
+    return rhs;
+  }();
+
+  const auto &shorter = [&] -> const BigInt & {
+    if (lhs.size() <= rhs.size()) {
+      return lhs;
+    }
+    return rhs;
+  }();
+
+  for (const auto &[bottomIdx, bottom] : vw::enumerate(shorter.digits)) {
+    u32 carry = 0;
+
+    for (const auto &[topIdx, top] : vw::enumerate(longer.digits)) {
+      u32 prod = bottom * top + carry;
+
+      carry = prod / Base;
+      prod %= Base;
+
+      accumulator[bottomIdx + topIdx] += prod;
+    }
+
+    if (carry != 0) {
+      accumulator[longer.size() + bottomIdx] += carry;
+    }
+  }
+
+  BigInt result;
+  u32 carry = 0;
+
+  result.digits.reserve(accumulator.size() + 2);
+
+  for (auto &prod : accumulator) {
+    prod += carry;
+
+    carry = prod / Base;
+    prod %= Base;
+
+    result.digits.push_back(prod);
+  }
+
+  while (carry != 0) {
+    result.digits.push_back(carry % Base);
+    carry /= Base;
+  }
+
+  result.trimLeadingZeroes();
+
+  return result;
+}
+
+std::strong_ordering BigInt::compareMagnitudes(const BigInt &lhs,
+                                               const BigInt &rhs) noexcept {
+  if (lhs.size() > rhs.size())
+    return std::strong_ordering::greater;
+
+  if (lhs.size() < rhs.size())
+    return std::strong_ordering::less;
+
+  for (const auto &[left, right] :
+       vw::zip(lhs.digits, rhs.digits) | vw::reverse) {
+    if (left > right)
+      return std::strong_ordering::greater;
+
+    if (left < right)
+      return std::strong_ordering::less;
+  }
+
+  return std::strong_ordering::equal;
+}
+
+BigInt operator+(const BigInt &lhs, const BigInt &rhs) {
+  if (lhs.isNegative == rhs.isNegative) {
+    BigInt result = BigInt::addMagnitudes(lhs, rhs);
+    result.isNegative = lhs.isNegative;
+    return result;
+  }
+
+  auto comparison = BigInt::compareMagnitudes(lhs, rhs);
+
+  if (comparison == std::strong_ordering::greater) {
+    BigInt result = BigInt::subMagnitudes(lhs, rhs);
+    result.isNegative = lhs.isNegative;
+    return result;
+  }
+
+  if (comparison == std::strong_ordering::less) {
+    BigInt result = BigInt::subMagnitudes(rhs, lhs);
+    result.isNegative = rhs.isNegative;
+    return result;
+  }
+
+  return BigInt{};
+}
+
+BigInt operator-(const BigInt &lhs, const BigInt &rhs) {
+  BigInt negated_rhs = rhs;
+
+  if (!negated_rhs.isZero()) {
+    negated_rhs.isNegative = !rhs.isNegative;
+  }
+
+  return lhs + negated_rhs;
+}
+
+BigInt operator*(const BigInt &lhs, const BigInt &rhs) {
+  BigInt result = BigInt::multiplyMagnitudes(lhs, rhs);
+
+  result.isNegative = lhs.isNegative ^ rhs.isNegative;
+
+  if (result.isZero()) {
+    result.isNegative = false;
+  }
+
+  return result;
+}
+
+std::expected<BigInt::DivisionResult, std::string>
+BigInt::divmod(const BigInt &lhs, const BigInt &rhs) {
+  DivisionResult result{};
+
+  if (rhs.isZero()) {
+    return std::unexpected{"Division by zero"};
+  }
+
+  if (lhs.isZero()) {
+    return result;
+  }
+
+  if (BigInt::compareMagnitudes(lhs, rhs) == std::strong_ordering::less) {
+    result.remainder = lhs;
+    return result;
+  }
+
+  auto &quotient = result.quotient;
+  auto &remainder = result.remainder;
+
+  remainder.digits.reserve(rhs.size());
+
+  for (const auto &digit : vw::reverse(lhs.digits)) {
+    remainder.digits.insert(remainder.digits.begin(), digit);
+    remainder.trimLeadingZeroes();
+
+    if (BigInt::compareMagnitudes(remainder, rhs) !=
+        std::strong_ordering::less) {
+      i16 quotDInt = 0;
+      BigInt quotD{};
+
+      for (i16 iter = Base - 1; iter >= 0; --iter) {
+        auto bigIter = BigInt::fromInteger(iter);
+
+        if (!bigIter) {
+          return std::unexpected{bigIter.error()};
+        }
+
+        auto mult = BigInt::multiplyMagnitudes(rhs, bigIter.value());
+
+        if (BigInt::compareMagnitudes(mult, remainder) !=
+            std::strong_ordering::greater) {
+          quotDInt = iter;
+          quotD = std::move(mult);
+          break;
+        }
+      }
+
+      remainder = BigInt::subMagnitudes(remainder, quotD);
+      quotient.digits.push_back(quotDInt);
+    }
+  }
+
+  rg::reverse(quotient.digits);
+  quotient.trimLeadingZeroes();
+
+  if (!quotient.isZero()) {
+    quotient.isNegative = lhs.isNegative ^ rhs.isNegative;
+  }
+
+  if (!remainder.isZero()) {
+    remainder.isNegative = lhs.isNegative;
+  }
+
+  return result;
+}
+
+std::expected<BigInt, std::string> BigInt::quotient(const BigInt &lhs,
+                                                    const BigInt &rhs) {
+  auto result = BigInt::divmod(lhs, rhs);
+
+  if (result) {
+    return result->quotient;
+  }
+
+  return std::unexpected(result.error());
+}
+
+std::expected<BigInt, std::string> BigInt::remainder(const BigInt &lhs,
+                                                     const BigInt &rhs) {
+  auto result = BigInt::divmod(lhs, rhs);
+
+  if (result) {
+    return result->remainder;
+  }
+
+  return std::unexpected(result.error());
+}
+
+std::strong_ordering operator<=>(const BigInt &lhs, const BigInt &rhs) {
+  if (lhs.isZero() && rhs.isZero()) {
+    return std::strong_ordering::equal;
+  }
+
+  if (lhs.isNegative && !rhs.isNegative) {
+    return std::strong_ordering::less;
+  }
+
+  if (!lhs.isNegative && rhs.isNegative) {
+    return std::strong_ordering::greater;
+  }
+
+  auto magnitude_comparison = BigInt::compareMagnitudes(lhs, rhs);
+
+  // If both are negative, the ordering is the reverse of their magnitude.
+  // e.g., magnitude of -100 > magnitude of -90, but -100 < -90.
+  if (lhs.isNegative) {
+    if (magnitude_comparison == std::strong_ordering::less)
+      return std::strong_ordering::greater;
+
+    if (magnitude_comparison == std::strong_ordering::greater)
+      return std::strong_ordering::less;
+
+    return std::strong_ordering::equal;
+  }
+
+  return magnitude_comparison;
+}
+
+bool operator==(const BigInt &lhs, const BigInt &rhs) {
+  return (lhs <=> rhs) == std::strong_ordering::equal;
+}
+
+bool operator!=(const BigInt &lhs, const BigInt &rhs) {
+  return !(lhs == rhs);
+}

common/bigint.hpp

@@ -0,0 +1,86 @@
+#pragma once
+
+#include <expected>
+#include <string>
+#include <string_view>
+#include <vector>
+
+#include <strings.hpp>
+#include <types.hpp>
+
+struct BigInt {
+  static constexpr u32 Base = 100;
+
+  bool isNegative = false;
+  std::vector<u8> digits;
+
+  struct DivisionResult;
+
+  friend BigInt operator+(const BigInt &, const BigInt &);
+  friend BigInt operator-(const BigInt &, const BigInt &);
+  friend BigInt operator*(const BigInt &, const BigInt &);
+  friend std::strong_ordering operator<=>(const BigInt &lhs, const BigInt &rhs);
+
+  static std::expected<BigInt, std::string>
+  fromString(std::string_view value);
+
+  static std::expected<BigInt, std::string>
+  fromInteger(std::integral auto value) {
+    auto str = std::to_string(value);
+    return BigInt::fromString(str);
+  }
+
+  std::string toString() const;
+
+  inline bool isZero() const noexcept {
+    return digits.empty();
+  }
+
+  BigInt() = default;
+  BigInt(BigInt &&) = default;
+  BigInt(const BigInt &) = default;
+  BigInt &operator=(BigInt &&) = default;
+  BigInt &operator=(const BigInt &) = default;
+  ~BigInt() = default;
+
+  static std::expected<DivisionResult, std::string> divmod(const BigInt &lhs,
+                                                           const BigInt &rhs);
+
+  static std::expected<BigInt, std::string> quotient(const BigInt &lhs,
+                                                     const BigInt &rhs);
+
+  static std::expected<BigInt, std::string> remainder(const BigInt &lhs,
+                                                      const BigInt &rhs);
+
+private:
+  void trimLeadingZeroes() noexcept;
+
+  inline u64 size() const noexcept {
+    return this->digits.size();
+  }
+
+  static BigInt addMagnitudes(const BigInt &lhs, const BigInt &rhs);
+
+  static BigInt subMagnitudes(const BigInt &lhs, const BigInt &rhs);
+
+  static BigInt multiplyMagnitudes(const BigInt &lhs, const BigInt &rhs);
+
+  static std::strong_ordering compareMagnitudes(const BigInt &lhs,
+                                                const BigInt &rhs) noexcept;
+};
+
+struct BigInt::DivisionResult {
+  BigInt quotient;
+  BigInt remainder;
+};
+
+BigInt operator+(const BigInt &lhs, const BigInt &rhs);
+
+BigInt operator-(const BigInt &lhs, const BigInt &rhs);
+
+BigInt operator*(const BigInt &lhs, const BigInt &rhs);
+
+std::strong_ordering operator<=>(const BigInt &lhs, const BigInt &rhs);
+
+bool operator==(const BigInt &lhs, const BigInt &rhs);
+bool operator!=(const BigInt &lhs, const BigInt &rhs);

common/strings.hpp

@@ -0,0 +1,44 @@
+#pragma once
+
+#include <cctype>
+#include <functional>
+#include <string_view>
+
+static inline bool isSpace(char ch) {
+  return std::isspace(ch);
+}
+
+static std::string_view ltrim(std::string_view str,
+                              std::function<bool(char)> filter = isSpace) {
+  auto it = str.begin();
+
+  while (it != str.end()) {
+    if (!filter(*it)) {
+      break;
+    }
+
+    ++it;
+  }
+
+  return {it, str.end()};
+}
+
+static std::string_view rtrim(std::string_view str,
+                              std::function<bool(char)> filter = isSpace) {
+  auto it = str.rbegin();
+
+  while (it != str.rend()) {
+    if (!filter(*it)) {
+      break;
+    }
+
+    ++it;
+  }
+
+  return {str.begin(), it.base()};
+}
+
+static std::string_view trim(std::string_view str,
+                             std::function<bool(char)> filter = isSpace) {
+  return ltrim(rtrim(str, filter), filter);
+}

common/types.hpp

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <cstdint>
+#include <ranges>
+
+using u8 = uint8_t;
+using u16 = uint16_t;
+using u32 = uint32_t;
+using u64 = uint64_t;
+
+using i8 = int8_t;
+using i16 = int16_t;
+using i32 = int32_t;
+using i64 = int64_t;
+
+using f32 = float;
+using f64 = double;
+
+namespace rg = std::ranges;
+namespace vw = std::views;