diff --git a/day2/p1.c b/day2/p1.c
index 522b0e3..58ad326 100644
--- a/day2/p1.c
+++ b/day2/p1.c
@@ -5,7 +5,7 @@
 #include <stdint.h>
 
 
-uint64_t parse_number(char *end);
+inline uint64_t parse_number(const char *end);
 
 #define SETS_STR_MAX_LEN 200
 
@@ -66,7 +66,7 @@ int main(int argc, char **argv) {
     return 0;
 }
 
-uint64_t parse_number(char *end) {
+inline uint64_t parse_number(const char *end) {
     uint64_t num = 0;
 
     for (size_t n = 1;; n *= 10) {
diff --git a/day2/p2.c b/day2/p2.c
index b850dc0..e9747a7 100644
--- a/day2/p2.c
+++ b/day2/p2.c
@@ -5,7 +5,7 @@
 #include <stdint.h>
 
 
-uint64_t parse_number(char *end);
+inline uint64_t parse_number(const char *end);
 
 #define SETS_STR_MAX_LEN 200
 
@@ -72,7 +72,7 @@ int main(int argc, char **argv) {
     return 0;
 }
 
-uint64_t parse_number(char *end) {
+inline uint64_t parse_number(const char *end) {
     uint64_t num = 0;
 
     for (size_t n = 1;; n *= 10) {
diff --git a/day3/p2.c b/day3/p2.c
index bead4db..1e5002c 100644
--- a/day3/p2.c
+++ b/day3/p2.c
@@ -7,7 +7,7 @@
 
 #define ISDIGIT(c) (c >= 0x30 && c <= 0x39)
 
-uint64_t parse_number(char *line, ssize_t j);
+inline uint64_t parse_number(const char *line, ssize_t j);
 
 
 int main(int argc, char **argv) {
@@ -101,7 +101,7 @@ int main(int argc, char **argv) {
     return 0;
 }
 
-uint64_t parse_number(char *line, ssize_t j) {
+inline uint64_t parse_number(const char *line, ssize_t j) {
     uint64_t n = 0;
     uint64_t multiplier = 1;
 
diff --git a/day4/p1.c b/day4/p1.c
index ba08bd9..378d1d7 100644
--- a/day4/p1.c
+++ b/day4/p1.c
@@ -5,7 +5,7 @@
 #include <stdint.h>
 
 
-uint64_t parse_number(char *end);
+inline uint64_t parse_number(const char *end);
 
 
 int main(int argc, char **argv) {
@@ -66,7 +66,7 @@ int main(int argc, char **argv) {
     return 0;
 }
 
-uint64_t parse_number(char *end) {
+inline uint64_t parse_number(const char *end) {
     uint64_t num = 0;
 
     for (size_t n = 1;; n *= 10) {
diff --git a/day4/p2.c b/day4/p2.c
index 62764bb..935fa48 100644
--- a/day4/p2.c
+++ b/day4/p2.c
@@ -8,7 +8,7 @@
 #define STACK_SZ UINT8_MAX+1
 typedef uint8_t stackpointer_t;
 
-uint64_t parse_number(char *end);
+inline uint64_t parse_number(const char *end);
 
 
 int main(int argc, char **argv) {
@@ -76,7 +76,7 @@ int main(int argc, char **argv) {
     return 0;
 }
 
-uint64_t parse_number(char *end) {
+inline uint64_t parse_number(const char *end) {
     uint64_t num = 0;
 
     for (size_t n = 1;; n *= 10) {
diff --git a/day5/p1.c b/day5/p1.c
index b2f454b..a1112b6 100644
--- a/day5/p1.c
+++ b/day5/p1.c
@@ -5,7 +5,7 @@
 #include <stdint.h>
 
 
-int64_t parse_number(char *end);
+inline int64_t parse_number(const char *end);
 
 
 int main(int argc, char **argv) {
@@ -69,7 +69,7 @@ int main(int argc, char **argv) {
     return 0;
 }
 
-int64_t parse_number(char *end) {
+inline int64_t parse_number(const char *end) {
     int64_t num = 0;
 
     for (size_t m = 1;; m *= 10) {
diff --git a/day5/p2.c b/day5/p2.c
index 754d1c0..5c3bfb3 100644
--- a/day5/p2.c
+++ b/day5/p2.c
@@ -5,7 +5,7 @@
 #include <stdint.h>
 
 
-uint64_t parse_number(char *end);
+inline uint64_t parse_number(const char *end);
 
 #define MAPS_LEN 7
 
@@ -100,7 +100,7 @@ int main(int argc, char **argv) {
     return 0;
 }
 
-uint64_t parse_number(char *end) {
+inline uint64_t parse_number(const char *end) {
     uint64_t num = 0;
 
     for (size_t m = 1;; m *= 10) {