Go strings Package

strings.Contains("gopher", "ph")       // true
strings.Contains("gopher", "xyz")      // false
strings.Contains("gopher", "")         // true — empty is always contained

// ContainsAny — true if any character in the set is present
strings.ContainsAny("hello", "aeiou")  // true — contains 'e' and 'o'
strings.ContainsAny("rhythm", "aeiou") // false — no vowels

// ContainsRune — check for a specific Unicode code point
strings.ContainsRune("hello", 'o')     // true

strings.HasPrefix("Gopher", "Go")      // true
strings.HasPrefix("Gopher", "go")      // false — case-sensitive

strings.HasSuffix("Amigo", "go")       // true
strings.HasSuffix("image.png", ".png") // true — handy for file extensions

// Returns byte index of first match, or -1
strings.Index("chicken", "ken")        // 4
strings.Index("chicken", "xyz")        // -1

// LastIndex — find last occurrence
strings.LastIndex("go gopher go", "go") // 10

// IndexAny — first position of any character in the set
strings.IndexAny("chicken", "aeiou")    // 2 — 'i'

// Count — non-overlapping occurrences
strings.Count("cheese", "e")            // 3
strings.Count("five", "")               // 5 — empty counts rune boundaries

// Split — all parts, separator removed
strings.Split("a,b,c", ",")            // ["a" "b" "c"]
strings.Split("hello", "")             // ["h" "e" "l" "l" "o"] — split each char

// SplitN — limit number of parts (last part holds remainder)
strings.SplitN("a,b,c,d", ",", 2)      // ["a" "b,c,d"]
strings.SplitN("a,b,c,d", ",", 3)      // ["a" "b" "c,d"]

// SplitAfter — keep the separator on the left piece
strings.SplitAfter("a,b,c", ",")       // ["a," "b," "c"]

// Fields — split on any whitespace, trims leading/trailing
strings.Fields("  foo bar  baz  ")     // ["foo" "bar" "baz"]

parts := []string{"hello", "world", "go"}

strings.Join(parts, ", ")              // "hello, world, go"
strings.Join(parts, " ")               // "hello world go"
strings.Join(parts, "")                // "helloworldgo"

// Common pattern: split then process then rejoin
words := strings.Split("one two three", " ")
result := strings.Join(words, "-")    // "one-two-three"

// Map applies a function to every rune in the string
rot13 := func(r rune) rune {
    switch {
    case r >= 'A' && r <= 'Z':
        return 'A' + (r-'A'+13)%26
    case r >= 'a' && r <= 'z':
        return 'a' + (r-'a'+13)%26
    }
    return r
}
strings.Map(rot13, "Hello, Gopher!")   // "Uryyb, Tbcure!"

// Drop all digits
noDigits := func(r rune) rune {
    if r >= '0' && r <= '9' { return -1 } // -1 drops the rune
    return r
}
strings.Map(noDigits, "a1b2c3")          // "abc"

// Trim — remove cutset chars from both ends
strings.Trim("¡¡¡Hello!!!", "!¡")          // "Hello"

// TrimLeft / TrimRight — one side only
strings.TrimLeft("¡¡¡Hello!!!", "!¡")      // "Hello!!!"
strings.TrimRight("¡¡¡Hello!!!", "!¡")     // "¡¡¡Hello"

// TrimSpace — removes all leading/trailing whitespace
strings.TrimSpace("  \t Hello, Go \n  ")   // "Hello, Go"

// Only removes the prefix/suffix if it matches exactly; no-op otherwise
strings.TrimPrefix("Goodbye, world!", "Goodbye, ") // "world!"
strings.TrimPrefix("Hello, world!", "Goodbye, ")  // "Hello, world!" — no match, unchanged

strings.TrimSuffix("image.png", ".png")             // "image"
strings.TrimSuffix("image.jpg", ".png")             // "image.jpg" — no match

// TrimFunc — trim using a custom predicate
strings.TrimFunc("  123abc456  ", unicode.IsSpace)  // "123abc456"

// Cut(s, sep) → before, after, found
before, after, found := strings.Cut("user@example.com", "@")
// before="user"  after="example.com"  found=true

before, after, found = strings.Cut("no-at-sign", "@")
// before="no-at-sign"  after=""  found=false

// Parsing key=value pairs
for _, pair := range []string{"host=localhost", "port=5432"} {
    key, val, _ := strings.Cut(pair, "=")
    fmt.Printf("%s → %s\n", key, val)
}
// host → localhost
// port → 5432

var b strings.Builder

for i := 0; i < 5; i++ {
    fmt.Fprintf(&b, "item%d ", i)
}
fmt.Println(b.String()) // "item0 item1 item2 item3 item4 "

// Other write methods
b.Reset()
b.WriteString("Hello")
b.WriteByte(',')
b.WriteRune(' ')
b.WriteString("World")
fmt.Println(b.String()) // "Hello, World"
fmt.Println(b.Len())    // 12

// Compare — lexicographic: returns -1, 0, or 1
strings.Compare("apple", "banana")  // -1  (apple < banana)
strings.Compare("banana", "apple")  // 1
strings.Compare("apple", "apple")   // 0

// EqualFold — case-insensitive equality (handles Unicode)
strings.EqualFold("Go", "go")       // true
strings.EqualFold("HELLO", "hello") // true
strings.EqualFold("Go", "Java")     // false

// Clone — explicit copy (avoids retaining a large backing array)
s := strings.Clone("hello")         // new allocation, same content