diff --git a/vlib/builtin/string.v b/vlib/builtin/string.v
index 8975f3d82a..8b6fa4b92e 100644
--- a/vlib/builtin/string.v
+++ b/vlib/builtin/string.v
@@ -2617,48 +2617,54 @@ pub fn (s string) camel_to_snake() string {
 	if s.len == 0 {
 		return ''
 	}
+	lower_first_char := if s[0] >= `A` && s[0] <= `Z` { s[0] + 32 } else { s[0] }
+	if s.len == 1 {
+		return lower_first_char.ascii_str()
+	}
+	mut b := unsafe { malloc_noscan(2 * s.len + 1) }
+	second_char := if s[1] >= `A` && s[1] <= `Z` { `_` } else { s[1] }
+	unsafe {
+		b[0] = lower_first_char
+		b[1] = second_char
+	}
+	mut prev_char := second_char
 	mut prev_is_upper := false
-	mut prev_char := ` `
 	mut lower_c := `_`
 	mut c_is_upper := false
-	mut b := unsafe { malloc_noscan(2 * s.len + 1) }
-	mut i := 0
-	for c in s {
+	mut pos := 1
+	for i in pos .. s.len {
+		c := s[i]
 		c_is_upper = c >= `A` && c <= `Z`
 		lower_c = if c_is_upper { c + 32 } else { c }
-		if i > 0 {
-			if prev_is_upper == false && c_is_upper {
-				// aB => a_b, if prev has `_`, then do not add `_`
-				unsafe {
-					if b[i - 1] != `_` {
-						b[i] = `_`
-						i++
-					}
+		if prev_is_upper == false && c_is_upper {
+			// aB => a_b, if prev has `_`, then do not add `_`
+			unsafe {
+				if b[pos - 1] != `_` {
+					b[pos] = `_`
+					pos++
 				}
-			} else if prev_is_upper && c_is_upper == false && c != `_` {
-				// Ba => _ba, if prev has `_`, then do not add `_`
-				if i > 1 {
-					unsafe {
-						if b[i - 2] != `_` {
-							prev_char = b[i - 1]
-							b[i - 1] = `_`
-							b[i] = prev_char
-							i++
-						}
-					}
+			}
+		} else if prev_is_upper && c_is_upper == false && c != `_` {
+			// Ba => _ba, if prev has `_`, then do not add `_`
+			unsafe {
+				if b[pos - 2] != `_` {
+					prev_char = b[pos - 1]
+					b[pos - 1] = `_`
+					b[pos] = prev_char
+					pos++
 				}
 			}
 		}
 		unsafe {
-			b[i] = lower_c
+			b[pos] = lower_c
 		}
 		prev_is_upper = c_is_upper
-		i++
+		pos++
 	}
 	unsafe {
-		b[i] = 0
+		b[pos] = 0
 	}
-	return unsafe { tos(b, i) }
+	return unsafe { tos(b, pos) }
 }
 
 // snake_to_camel convert string from snake_case to camelCase
diff --git a/vlib/builtin/string_test.v b/vlib/builtin/string_test.v
index 09661a69f1..2ab4a6cf73 100644
--- a/vlib/builtin/string_test.v
+++ b/vlib/builtin/string_test.v
@@ -1525,6 +1525,7 @@ fn test_contains_byte() {
 fn test_camel_to_snake() {
 	assert 'Abcd'.camel_to_snake() == 'abcd'
 	assert 'aaBB'.camel_to_snake() == 'aa_bb'
+	assert 'aaBbCcDD'.camel_to_snake() == 'aa_bb_cc_dd'
 	assert 'BBaa'.camel_to_snake() == 'b_baa'
 	assert 'aa_BB'.camel_to_snake() == 'aa_bb'
 }
@@ -1532,6 +1533,7 @@ fn test_camel_to_snake() {
 fn test_snake_to_camel() {
 	assert 'abcd'.snake_to_camel() == 'Abcd'
 	assert 'ab_cd'.snake_to_camel() == 'AbCd'
+	assert 'ab_cd_efg'.snake_to_camel() == 'AbCdEfg'
 	assert '_abcd'.snake_to_camel() == 'Abcd'
 	assert '_abcd_'.snake_to_camel() == 'Abcd'
 }