Karl:

options(na.action=na.exclude) # preserve missings
options(contrasts=c('contr.treatment', 'contr.poly')) #ensure constrast type
library(survival)

#
# The test data set from Turnbull, JASA 1974, 169-73.
#
#  status  0=right censored
#          1=exact
#          2=left censored
#
aeq <- function(x,y, ...) all.equal(as.vector(x), as.vector(y), ...)


turnbull <- data.frame( time  =c( 1,1,1, 2,2,2, 3,3,3, 4,4,4),
			status=c( 1,0,2, 1,0,2, 1,0,2, 1,0,2),
			  n   =c(12,3,2, 6,2,4, 2,0,2, 3,3,5))
#
# Compute the K-M for the Turnbull data
#      via a slow EM calculation
#

emsurv <- function(time, status, wt, verbose=T) {
    left.cen <- (status==2)
    if (!any(left.cen)) stop("No left censored data!")
    if (!any(status==1))stop("Must have some exact death times")

    tempy <- Surv(time[!left.cen], status[!left.cen])
    ww <- wt[!left.cen]
    tempx <- factor(rep(1, sum(!left.cen)))
    tfit <- survfit(tempy~tempx, weight=ww)
    if (verbose)
       cat("Iteration 0, survival=", format(round(tfit$surv[tfit$n.event>0],3)),
		       "\n")

    stimes <- tfit$time[tfit$n.event>0]
    ltime <- time[left.cen]
    lwt   <- wt[left.cen]
    tempx <- factor(rep(1, length(stimes) + sum(!left.cen)))
    tempy <- Surv(c(time[!left.cen], stimes),
		  c(status[!left.cen], rep(1, length(stimes))))
    for (iter in 1:4) {
	wt2 <- stimes*0
	ssurv <- tfit$surv[tfit$n.event>0]
	sjump <- diff(c(1, ssurv))
	for (j in 1:(length(ltime))) {
	    k <- sum(ltime[j]>=stimes)   #index of the death time
	    if (k==0)
		stop("Left censored observation before the first death")
	    wt2[1:k] <- wt2[1:k] + lwt[j]*sjump[1:k] /(ssurv[k]-1)
	    }
	tfit <- survfit(tempy~tempx, weight=c(ww, wt2))
	if (verbose) {
	   cat("Iteration", iter, "survival=",
		 format(round(tfit$surv[tfit$n.event>0],3)),  "\n")
	   cat("             weights=", format(round(wt2,3)), "\n")
	   }
	}
    survfit(tempy ~ tempx, weights=c(ww, wt2))
    }

temp <-emsurv(turnbull$time, turnbull$status, turnbull$n)
print(summary(temp))
# First check, use the data from Turnbull, JASA 1974, 169-173.

tdata <- data.frame(time  =c(1,1,1,2,2,2,3,3,3,4,4,4),
		    status=rep(c(1,0,2),4),
		    n     =c(12,3,2,6,2,4,2,0,2,3,3,5))

tfit <- survfit(Surv(time, time, status, type='interval') ~1, tdata, weight=n)
all.equal(round(tfit$surv,3), c(.538, .295, .210, .095))


# Second check, compare to a reversed survival curve
# This is not as simple a test as one might think, because left and right
#  censored observations are not treated symmetrically by the routine:
#  time <= y for left and time> y for right (this is to make the routine
#  correct for the common situation of panel data).
# To get equivalence, make the left censoreds happen just a little bit
#  earlier.  The left-continuous/right-continuous shift is also a bother.
#
test1 <- data.frame(time=  c(9, 3,1,1,6,6,8),
                    status=c(1,NA,1,0,1,1,0),
                    x=     c(0, 2,1,1,1,0,0))
fit1 <- survfit(Surv(time, status) ~1, test1)
temp <- ifelse(test1$status==0, 4.99,5) - test1$time
fit2 <- survfit(Surv(temp, status, type='left') ~1, test1)

all.equal(round(fit1$surv[1:2],5), round(1-fit2$surv[3:2],5))

rm(tdata, tfit, fit1, temp, fit2)
#
# Create a data set similar to the one provided by Al Zinsmeister
#  It is a hard test case for survfit.turnbull
#
time1 <- c(rep(0,100), rep(1,200), 100, 200, 210, 220,
           rep(365,100), rep(366,5), 731:741)

time2 <- c((1:100)*3,  10+1:100, rep(365:366, c(60,40)), NA, 500, NA, 450,
           rep(730,90), rep(NA,10), c(528,571,691,730,731),
           NA, 1095:1099, NA, 1400, 1200, 772, 1461)

zfit <- survfit(Surv(time1, time2, type='interval2') ~1)

#
# There are 100 intervals of the form (0,x) where x is from 3 to 300,
#  and 200 more of the form (1,x) where x is from 11 to 366.  These
#  lead to a mass point in the interval (1,3), which is placed at 2.
#  The starting estimate has far too little mass placed here, and it takes
#  the EM a long time to realize that most of the weight for the first 300
#  subjects goes here.  With acceleration, it takes 16 iterations, without
#  it takes >40.  (On Al's orginal data, without accel still wasn't there after
#  165 iters!)
#
# The next 4 obs give rise to potential jumps at 100.5, 200.5, 211.5, and
#  221.  However, the final estimate has no mass at all on any of these.
#  Assume mass of a,b, and c at 2, 100.5 and 365.5, and consider the 
#  contributions: 
#    123 obs that overlap a only
#    137 obs that overlap a and b
#     40 obs that overlap a, b, c
#      1 obs that overlap b, c
#    108 obs that overlap c   (200, 210,200, 365, and 366 starting points)
#  For some trial values of a,b,c, compare the loglik to that of (a+b),0,c
#   First one: a^123 (a+b)^137 (a+b+c)^40 (b+c) c^108
#   Second:    (a+b)^123 (a+b)^137 (a+b+c)^40 c c^108
#   Likelhood improves if (1 + b/a)^123 > 1+ b/c, which is true for almost
#     all a and c.  In particular, at the solution a and c are approx .7 and
#     .18, respectively.
#
# The program can't see this coming, of course, and so iterates towards a
#  KM with epsilon sized jumps at 100.5, 200.5, and 211.5.  Whether these
#  intervals should be removed during iteration, as detected, is an open
#  question for me.
#
#
# True solution: mass points at 2, 365.5, 408, and 756.5, of sizes a, b, c, d
# Likelihood:    a^260 (a+b)^40 (b+c)^92 (b+c+d)^12 c^5 d^11
# Solution: a=0.6958, b=0.1674, c=0.1079, d=0.0289

tfun <- function(x) {
    if (length(x) ==3) x <- c(x, .03)
    x <- x/sum(x)  #make probabilities sum to 1
    loglik <- 260*log(x[1]) + 40*log(x[1]+x[2]) + 92*log(x[2] + x[3]) +
                       12*log(x[2]+x[3]+x[4]) + 5*log(x[3]) + 11*log(x[4])
    -loglik  #find the max, not the min
    }

nfit <- nlminb(start=c(.7,.15, .1), tfun, lower=0, upper=1)
nparm <- c(nfit$par, .03)
nparm <- nparm / sum(nparm)
zparm <- -diff(c(1, zfit$surv[match(c(2, 365.5, 408, 756.5), zfit$time)]))
aeq(round(tfun(nparm),4), round(tfun(zparm),4))
# .0001 is the tolerance in survfit.turnbull

rm(tfun, nfit, nparm, zparm, time1, time2, zfit)