# Generic negative exponential function with offset
#  y = c (1 - exp( exp(-b) (x - a) ) )
#  
# ... modified from SSasympOff
# 16-Feb-2008
#
Ftaf <- function (t, s2, s3, s4, b, a,  dc2, dc3, dc4, db2, db3, db4, da2, da3, da4) {

c <- 100

# model
.expr1 <- exp(b + db2*s2 + db3*s3 + db4*s4)
.expr3 <- t - (a + da2*s2 + da3*s3 + da4*s4)
.expr5 <- exp(((-.expr1) * .expr3))
.expr6 <- 1 - .expr5
.value <- (c + dc2*s2 + dc3*s3 + dc4*s4)*.expr6

# set gradient attribute
.actualArgs <- as.list(c( "b", "a", "dc2", "dc3", "dc4", "db2", "db3", "db4", "da2", "da3", "da4" ))

.grad <- array(0, c(length(.value), 11), list(NULL, c( "b", "a", "dc2", "dc3", "dc4", "db2", "db3", "db4", "da2", "da3", "da4" )))

.grad[, "b"]  <-  c * (.expr5 * (.expr1 * .expr3))
.grad[, "a"]  <-  -(c * (.expr5 * .expr1))

.grad[, "dc2"] <- ifelse(s2, .expr6, 0)
.grad[, "dc3"] <- ifelse(s3, .expr6, 0)
.grad[, "dc4"] <- ifelse(s4, .expr6, 0)

.grad[, "db2"] <- ifelse(s2, c * (.expr5 * (.expr1 * .expr3)), 0)
.grad[, "db3"] <- ifelse(s3, c * (.expr5 * (.expr1 * .expr3)), 0)
.grad[, "db4"] <- ifelse(s4, c * (.expr5 * (.expr1 * .expr3)), 0)

.grad[, "da2"] <- ifelse(s2, -(c * (.expr5 * .expr1)), 0)
.grad[, "da3"] <- ifelse(s3, -(c * (.expr5 * .expr1)), 0)
.grad[, "da4"] <- ifelse(s4, -(c * (.expr5 * .expr1)), 0)

dimnames(.grad) <- list(NULL, .actualArgs)
attr(.value, "gradient") <- .grad

# return
.value
}