Length of a UTF-8 byte sequence

Z80Golf, 19 14 bytes

00000000: 2f6f 3e10 37ed 6a3d 30fb ee07 c03c       /o>.7.j=0....<

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-5 bytes thanks to @Bubbler

Example with input 0x41-Try it online! Assembly

Example with input 0xC2-Try it online!

Example with input 0xE0-Try it online!

Example with input 0xF4-Try it online!

Assembly:

;input: register a
;output: register a
byte_count:			;calculate 7^(log2(255^a))||1
	cpl			;xor 255
	ld l,a
	log2:
		ld	a,16
		scf
	log2loop:
		adc	hl,hl
		dec	a
		jr	nc,log2loop
	xor 7
	ret nz
	inc a

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Forth, 6 bytes

x-size

see https://forth-standard.org/standard/xchar/X-SIZE

Input and output follows a standard Forth model:

Input

Memory address + length (i.e. 1) of a single-byte UTF-8 "string".

Output

UTF-8 sequence length in bytes.

Sample Code

Store 0xF0 in a memory cell, and invoke x-size:

variable v
0xF0 v !
v 1 x-size

Check the result:

.s <1> 4  ok

C (gcc), 39 bytes

t(char x){x=(__builtin_clz(~x)-24)%7u;}

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