Simplex v.0.5, 12 bytes

(The Docs page may be outdated; mainly, the * also increments the pointer.)

h*M{*LTRpM}]

This defines a macro that performs the factorial function on the current byte. It maintains the structure of the strip, but inserts an extra 0 at the next byte. You can delete this by adding another command p before the function ends. Here the non-destructive version being used in an example code:

h*M{*LTRpM}p]ih0o

This defines the macro, asks for numeric input (i), calls the first macro (h0) and outputs the byte as a number (o).

Here is the pseudo-code I used:

Function factorial(N)
    A = N - 1
    While A > 1
        N = A * N
        A = A - 1
    End While
    Return N
End Function

This is the expanded explanation.

h    ~~ open macro, implicit [
 *   ~~ A=N [N,A]
 M   ~~ A=N-1 [N,A-1]
 {   ~~ Loop until current byte is zero
  *  ~~ [N,A-1,A-1]
  LT ~~ [N*(A-1),0,A-1]
  Rp ~~ [N*(A-1),A-1]
  M  ~~ [N*(A-1),A-2]
 }
 p   ~~ [N!]
]    ~~ close macro

Simplex v.0.5, 12 bytes

(The Docs page may be outdated; mainly, the * also increments the pointer and the J being the max of two elements.)

h*M{*LTRpM}]

This defines a macro that performs the factorial function on the current byte. It maintains the structure of the strip, but inserts an extra 0 at the next byte. You can delete this by adding another command p before the function ends.

This one works for inputs on a strip whose sole member is the input. I.e., a strip which looks like [N,/,/,...] (/ is the empty or null bit.) It clocks in at…

11 Bytes!!

This beats the GolfScript entry, FYI.

h{*M}pwT1J]

This is what it does:

h{*M}pwT1J]
h         ] ~~ define new macro
 {  }       ~~ repeat inside until zero met
  *         ~~ copy the current byte and increment pointer
   M        ~~ decrement byte
     p      ~~ remove trailing zero
      wT    ~~ spreads T (multiplication) across strip backwards; sets pointer to after the result
        1J  ~~ Takes the maximum of 1 and the current byte

Here the non-destructive version being used in an example code:

h*M{*LTRpM}p]ih0o

This defines the macro, asks for numeric input (i), calls the first macro (h0) and outputs the byte as a number (o).

Here is the pseudo-code I used:

Function factorial(N)
    A = N - 1
    While A > 1
        N = A * N
        A = A - 1
    End While
    Return N
End Function

This is the expanded explanation.

h    ~~ open macro, implicit [
 *   ~~ A=N [N,A]
 M   ~~ A=N-1 [N,A-1]
 {   ~~ Loop until current byte is zero
  *  ~~ [N,A-1,A-1]
  LT ~~ [N*(A-1),0,A-1]
  Rp ~~ [N*(A-1),A-1]
  M  ~~ [N*(A-1),A-2]
 }
 p   ~~ [N!]
]    ~~ close macro

Simplex v.0.5, 12 bytes

(The Docs page may be outdated; mainly, the * also increments the pointer.)

h*M{*LTRpM}]

This defines a macro that performs the factorial function on the current byte. It maintains the structure of the strip.

Here is the pseudo-code I used:

Function factorial(N)
    A = N - 1
    While A > 1
        N = A * N
        A = A - 1
    End While
    Return N
End Function

This is the expanded explanation.

h
 *   ~~ A=N [N,A]
 M   ~~ A=N-1 [N,A-1]
 {
  *  ~~ [N,A-1,A-1]
  LT ~~ [N*(A-1),0,A-1]
  Rp ~~ [N*(A-1),A-1]
  M  ~~ [N*(A-1),A-2]
 }
 p   ~~ [N!]
]

Simplex v.0.5, 12 bytes

(The Docs page may be outdated; mainly, the * also increments the pointer.)

h*M{*LTRpM}]

This defines a macro that performs the factorial function on the current byte. It maintains the structure of the strip, but inserts an extra 0 at the next byte. You can delete this by adding another command p before the function ends. Here the non-destructive version being used in an example code:

h*M{*LTRpM}p]ih0o

This defines the macro, asks for numeric input (i), calls the first macro (h0) and outputs the byte as a number (o).

Here is the pseudo-code I used:

Function factorial(N)
    A = N - 1
    While A > 1
        N = A * N
        A = A - 1
    End While
    Return N
End Function

This is the expanded explanation.

h    ~~ open macro, implicit [
 *   ~~ A=N [N,A]
 M   ~~ A=N-1 [N,A-1]
 {   ~~ Loop until current byte is zero
  *  ~~ [N,A-1,A-1]
  LT ~~ [N*(A-1),0,A-1]
  Rp ~~ [N*(A-1),A-1]
  M  ~~ [N*(A-1),A-2]
 }
 p   ~~ [N!]
]    ~~ close macro